Rebase of #12911

This adds a "fee" field to the resulting JSON for signrawtransaction* so a user can double check the fee they're paying before sending a transaction. The field is only shown in cases where the input amounts are all known ⇔ are all segwit inputs.

$ ./bitcoin-cli -regtest signrawtransactionwithwallet 0200000001901c16d5ac11824ca64c9c9dbe925c83fc1af9872bb23bbc9c6cd25419e2f69c0000000000feffffff0210b2d0df0000000017a914d9214ccd777e5cce540d38c3466c2cb5545339c5874031354a0000000017a914bee793caf793996dc9f617a5ad04ec2b87c6f9538700000000
{
  "hex": "0200000001901c16d5ac11824ca64c9c9dbe925c83fc1af9872bb23bbc9c6cd25419e2f69c00000000494830450221008fd0d0cfd16a06f282e720129351f2756f416b916f7a0a3be8d5db0c7db107af022028dafae6ec7d30882efe101c16b5f3893254bf5385664eddadf0d3f6e479381c01feffffff0210b2d0df0000000017a914d9214ccd777e5cce540d38c3466c2cb5545339c5874031354a0000000017a914bee793caf793996dc9f617a5ad04ec2b87c6f9538700000000",
  "complete": true,
  "fee": 0.00003760,
  "feerate": 0.00020000
}