feat(postgrest): support INSERT ... RETURNING for insert().select()

[MuzzaiyyanHussain]

Description

Fixes #1832 — Enables .insert(...).select('id') to work without requiring SELECT RLS policy
by using INSERT ... RETURNING instead of an additional SELECT request.

Changes

• Automatically applies Prefer: return=representation to insert() requests.
• Ensures backward compatibility for all existing insert flows.

Impact

• Users with only INSERT permission can now retrieve IDs of inserted rows.
• Reduces insert+select to a single network call.

Checklist

• Build passes
• Verified correct Prefer header behavior
• Closes On an INSERT with a SELECT, use RETURNING (Instead of a separate SELECT call) #1832

[coveralls]

coverage: 95.367% (+13.8%) from 81.58%
when pulling 1dd6b8e on MuzzaiyyanHussain:feat-insert-returning
into 1371057 on supabase:master.

[steve-chavez]

Not sure if this feature is right, I've asked for more input on #1832 (comment)