use strings for db type everywhere

[matt2e]

@worstell's new PR uses strings for db types #1374
Previously I had added a db type for modulecontext which was an int. We should move everything to use strings under the hood.

[mohitsethia]

Hi @matt2e @worstell , I would like to contribute to this, can you please help a bit as I am new here.

[matt2e]

@mohitsethia here's some pointers that might help:

• We added this type to our grpc proto to model a database type: https://github.com/TBD54566975/ftl/blob/main/backend/protos/xyz/block/ftl/v1/ftl.proto#L31
  • This is used when runners ask the controller for ModuleContext (secrets, config and DSNs)
• We also added a string type for modeling a database's type here: https://github.com/TBD54566975/ftl/blob/main/backend/schema/database.go#L12
  • This is used when modeling our module schemas

This task is to try and move the first type to being a string like the second type and to use the same string values