Allow instances for createaggregatefunction

[rogerbinns]

Currently it only allows methods to be separately returned. Copy create_window_function which allows an instance, or the methods separately

• [x] Update C code
• [x] Update doc
• [x] Update example code
• [x] Update type stubs
• [x] Update changes doc
• [x] Add test code
• [x] Verify fault injection and debug build