feat(types): export GenericSchema type from types

[brunorodmoreira]

What kind of change does this PR introduce?

Add the export of GenericSchema in order to allow to use it as base to Database type argument in the PostgrestClient.

What is the current behavior?

The type is not exported, so if we are using the client without the generated types, we are not able to get type hints.

What is the new behavior?

Exporting it, we can extend it and get type hints.

Additional context

Expected behavior: