spannertest: support JSON type

**Is your feature request related to a problem? Please describe.**
`spannertest` currently does not support the JSON type. As we expect to shift our testing left and have comprehensive unit tests against our schemas and DB related code (using the `spannertest` package), this prevents us from using the JSON type at all in our code with Spanner.

**Describe the solution you'd like**
`spannertest` should support the JSON type for columns, along with the spanner client operations and APIs for them.

**Describe alternatives you've considered**
* Not unit testing (not a viable option for us)
* Storing JSON as string instead (subpar and even less efficient as it requires parsing to JSON in order to make use of the structure)

**Additional context**
This has been a gap in `spannertest` for a long time and it would be great if it can finally be supported.