feat: add PostgREST retry option to client

[Niveditha251006]

What does this PR do?

Adds a client-level option to disable automatic retries for transient PostgREST errors.

Changes

• Adds postgrest_client_retry to ClientOptions
• Propagates the option through sync and async Supabase clients
• Propagates the setting to PostgREST request builders
• Preserves the default retry behavior (True)
• Adds sync and async tests
• Documents the option in the Supabase README

Testing

• 86 PostgREST request-builder tests passed
• Sync retry-option test passed
• Async retry-option test passed

Closes #1561