fix(realtime): add generic overload for postgres_changes event type

[7ttp]

Adds a generic overload for on() with postgres_changes that accepts any event type, enabling reusable subscription helpers with union types like '*' | 'INSERT' | 'UPDATE' | 'DELETE'.

Fixes "No overload matches this call" error when passing generic event parameters to channel.on('postgres_changes', ...).

closes #1451

[coveralls]

coverage: 95.349% (+14.0%) from 81.389%
when pulling eaf16ef on 7ttp:fix/realtime11
into 30f9600 on supabase:master.

[mandarini]

@7ttp thank you for this PR. Can you please add a test for the new functionality? It can go in packages/core/realtime-js/test/RealtimeChannel.postgres.test.ts. Something like:

...
      const event: '*' | 'INSERT' | 'UPDATE' | 'DELETE' = 'INSERT'
      // This should compile without errors (previously required `as any`)
      channel.on(
        'postgres_changes',
        {
          event,
          schema: 'public',
          table: 'users',
        },
...
      )
...
      expect(channel.bindings.postgres_changes.length).toBe(1)
...

[mandarini]

Requested test!