feat: Add support for mocking RPC functions.

[dshukertjr]

What kind of change does this PR introduce?

Add RPC support.

What is the current behavior?

No RPC support

What is the new behavior?

Users can now mock RPC functions through the registerRpcFunction function.

 mockSupabaseHttpClient.registerRpcFunction(
   'get_status',
   (params, tables) => {'status': 'ok'},
 );

 final mockSupabase = SupabaseClient(
   'https://mock.supabase.co',
   'fakeAnonKey',
   httpClient: mockSupabaseHttpClient,
 );

 mockSupabase.rpc('get_status').select(); // returns {'status': 'ok'}

Example of an RPC function that modifies the data in the database:

mockSupabaseHttpClient.registerRpcFunction(
  'update_post_title',
  (params, tables) {
    final postId = params!['id'] as int;
    final newTitle = params!['title'] as String;
    final post = tables['public.posts']!.firstWhere((post) => post['id'] == postId);
    post['title'] = newTitle;
  },
);

final mockSupabase = SupabaseClient(
  'https://mock.supabase.co',
  'fakeAnonKey',
  httpClient: mockSupabaseHttpClient,
);

// Insert initial data
await mockSupabase.from('posts').insert([
  {'id': 1, 'title': 'Old title'},
]);

// Call the RPC function
await mockSupabase.rpc('update_post_title', params: {'id': 1, 'title': 'New title'});

// Verify that the post was modified
final posts = await mockSupabase.from('posts').select().eq('id', 1);
expect(posts.first['title'], 'New title');