feat: add json output exposing the GeneratorMetadata contract

[spydon]

Summary

Reopens #1110 (auto-closed when its base branch merged as #1084 and was deleted), rebased onto master.

Adds a json output to type generation, exposing the raw GeneratorMetadata introspection contract of @supabase/postgrest-typegen for third-party type generators (for example supabase_typegen for Dart):

• GET /generators/json (with included_schemas/excluded_schemas)
• PG_META_GENERATE_TYPES=json for the CLI path (npm run gen:types:json)

Changes since the original #1110

The rebase updated the endpoint test for the contract shipped in v0.99.0: the document now carries version (asserted to be 1) and primaryKeys, both introduced by the extracted package's introspect().

Validation

• npm run check (tsc) and prettier --check pass.
• Both json endpoint tests pass against the test database, asserting content type, the full key set, introspection fidelity (nullability, defaults, identity), and schema filtering.