08fc6f6 [rpc] refactor: consolidate sendmany and sendtoaddress code (Sjors Provoost)

Pull request description:

  I consolidated code between these two RPC calls, since `sendtoaddress` is essentially `sendmany` with 1 destination.

  Unless I overlooked something, the only behaviour change is that some `sendtoaddress` error codes changed from `-4` to `-6`. The release note mentions this.

  Salvaged from #18201.

ACKs for top commit:
  fjahr:
    Code review ACK 08fc6f6
  jonatack:
    ACK 08fc6f6
  meshcollider:
    Code review & functional test run ACK 08fc6f6

Tree-SHA512: 7b66c52fa0444a4d02fc3f81d9c2a386794d447616026a30111eda35fb46510475eea6506a9ceda00bb4e0230ebb758da5d236b3ac05c954c044fa68a1e3e909

… code

08fc6f6 [rpc] refactor: consolidate sendmany and sendtoaddress code (Sjors Provoost)

Pull request description:

  I consolidated code between these two RPC calls, since `sendtoaddress` is essentially `sendmany` with 1 destination.

  Unless I overlooked something, the only behaviour change is that some `sendtoaddress` error codes changed from `-4` to `-6`. The release note mentions this.

  Salvaged from bitcoin#18201.

ACKs for top commit:
  fjahr:
    Code review ACK 08fc6f6
  jonatack:
    ACK 08fc6f6
  meshcollider:
    Code review & functional test run ACK 08fc6f6

Tree-SHA512: 7b66c52fa0444a4d02fc3f81d9c2a386794d447616026a30111eda35fb46510475eea6506a9ceda00bb4e0230ebb758da5d236b3ac05c954c044fa68a1e3e909

92326d8 [rpc] add send method (Sjors Provoost)
2c2a144 [rpc] add snake case aliases for transaction methods (Sjors Provoost)
1bc8d0f [rpc] walletcreatefundedpsbt: allow inputs to be null (Sjors Provoost)

Pull request description:

  `walletcreatefundedpsbt` has some interesting features that `sendtoaddress` and `sendmany` don't have:
  * manual coin selection
  * outputting a PSBT (it was controversial to add this, see #18201)
  * create a transaction without adding to wallet (which leads to broadcasting, unless `-walletbroadcast=0`)

  At the same time `walletcreatefundedpsbt` can't broadcast a transaction, which is inconvenient for simple use cases.

  This PR introduces a new `send` RPC method which creates a PSBT, signs it if possible and adds it to the wallet by default. If it can't sign all inputs, it outputs a PSBT. If `add_to_wallet` is set to `false` it will return the transaction in both PSBT and hex format.

  Because it uses a PSBT internally, it will much easier to add hardware wallet support to this method (see #16546).

  For `bitcoin-cli` users, it tries to keep the simplest use case easy to use:

  ```sh
  bitcoin-cli -regtest send '{"ADDRESS": 0.1}' 1 sat/b
  ```

  This paves the way for deprecating `sendtoaddress` and `sendmany` though there's no rush. The only missing feature compared to these older methods is adding labels to a destination address.

  Depends on:
  - [x] #16377 (`[rpc] don't automatically append inputs in walletcreatefundedpsbt`)
  - [x] #11413 (`[wallet] [rpc] sendtoaddress/sendmany: Add explicit feerate option`)
  - [x] #18244 (`[rpc] have lockUnspents also lock manually selected coins`)

ACKs for top commit:
  meshcollider:
    Light re-utACK 92326d8
  achow101:
    ACK 92326d8 Reviewed code and test, ran tests.
  kallewoof:
    utACK 92326d8

Tree-SHA512: 7552ef1b193d4c06e381c44932fdb0d54f64383e4c7d6b988f49d059c7d4bba45ce6aa7813e03df86360ad9dad6f3010eb76ee7da480551742d5fd98c2251c0f

92326d8 [rpc] add send method (Sjors Provoost)
2c2a144 [rpc] add snake case aliases for transaction methods (Sjors Provoost)
1bc8d0f [rpc] walletcreatefundedpsbt: allow inputs to be null (Sjors Provoost)

Pull request description:

  `walletcreatefundedpsbt` has some interesting features that `sendtoaddress` and `sendmany` don't have:
  * manual coin selection
  * outputting a PSBT (it was controversial to add this, see bitcoin#18201)
  * create a transaction without adding to wallet (which leads to broadcasting, unless `-walletbroadcast=0`)

  At the same time `walletcreatefundedpsbt` can't broadcast a transaction, which is inconvenient for simple use cases.

  This PR introduces a new `send` RPC method which creates a PSBT, signs it if possible and adds it to the wallet by default. If it can't sign all inputs, it outputs a PSBT. If `add_to_wallet` is set to `false` it will return the transaction in both PSBT and hex format.

  Because it uses a PSBT internally, it will much easier to add hardware wallet support to this method (see bitcoin#16546).

  For `bitcoin-cli` users, it tries to keep the simplest use case easy to use:

  ```sh
  bitcoin-cli -regtest send '{"ADDRESS": 0.1}' 1 sat/b
  ```

  This paves the way for deprecating `sendtoaddress` and `sendmany` though there's no rush. The only missing feature compared to these older methods is adding labels to a destination address.

  Depends on:
  - [x] bitcoin#16377 (`[rpc] don't automatically append inputs in walletcreatefundedpsbt`)
  - [x] bitcoin#11413 (`[wallet] [rpc] sendtoaddress/sendmany: Add explicit feerate option`)
  - [x] bitcoin#18244 (`[rpc] have lockUnspents also lock manually selected coins`)

ACKs for top commit:
  meshcollider:
    Light re-utACK 92326d8
  achow101:
    ACK 92326d8 Reviewed code and test, ran tests.
  kallewoof:
    utACK 92326d8

Tree-SHA512: 7552ef1b193d4c06e381c44932fdb0d54f64383e4c7d6b988f49d059c7d4bba45ce6aa7813e03df86360ad9dad6f3010eb76ee7da480551742d5fd98c2251c0f