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.