- Generate compliant DDL statements for creating Spanner tables
  from proto models
- Add ability to stub/mock `FieldPointer` objects

- Add `TypeBuffet` to test conversion of different types
- Add tests for `SpannerUtil`
- Add initial tests for Spanner DDL generator
- Uncomment more tests from `GenericPersistenceDriverTests`

- Allow test caching (local only)
- Enable other adapter/driver tests

- Fix CSS testing bugs
- Re-enable style tests
- Add suite targets for DOM/style tests

- Allow drivers to opt-out of tests they don't yet support
- Enable all tests across the board

- Add `SpannerDDLTest`
- Add `SpannerUtilTest`

- Implement singular delete method
- Enable standard driver `delete()` tests

- Enforce `FieldMask` settings for fetched records
- Drop superfluous error checking

- Fix bug with `ARRAY` fields missing inner types
- Add sensible default for `ENUM` fields implemented as `STRING`

- Enable column sizes for `BYTES` fields
- Transition long string comparisons to Truth assertions

- Generate tables for emulated testing
- Add method alias to generate DDL statements just from a model
  instance (with default settings)

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This changeset adds support for Spanner's new native JSON columns,
which essentially act as strings but with enforced JSON-friendly
semantics which improve queryability.

Spanner's JSON fields differ from strings in the following ways:
- Keys are sorted lexicographically
- Whitespace is not preserved
- Duplicate keys are elided
- Numeric and other primitive types are interpreted natively
- JSON `null` is treated as SQL `NON NULL`

These columns have advantages for various kinds of JSON document
storage based in Spanner. See docs here:

- [Spanner: Working with JSON](https://cloud.google.com/spanner/docs/working-with-json)
- [Spanner/J: Driver support PR](googleapis/java-spanner#872)

Changes enclosed:
- Add support to DDL generator for JSON columns
- Driver-side experimental flag to enable/disable
- Compliant with existing testsuite (fallback to `STRING`)
- Tests for tables with JSON columns
- Tests for DDL generator with JSON columns enabled

This changeset adds a fix for sgammon/elide#868, wherein default
`BOOL` column values should be the `false` value, rather than no
value at all. This PR implements the approach described in the
issue, gated behind a driver setting.

Fixes and closes sgammon/elide#868.

This changeset updates some key dependencies that have issued
newer releases since last pin.

- [x] Update GAX -> `2.7.0`
- [x] Update Google Cloud APIs -> `0.163.0`
- [x] Update common protos -> `2.6.0`

- Fallback to `TIMESTAMP` for `DATE` fields that error (seems to
  be a driver quirk)
- Allow decoding spanner `DATE` fields into standard
  `google.protobuf.Timestamp` types