Add extension modifier function for bot reply token stream

[Th-Underscore]

Basically just a bridge between output_modifier and custom_generate_reply

Also considered the idea of:

def output_stream_modifier(generator, state, is_chat=False):
    for output in generator(...):
        yield output

but I think it's pretty complicated to implement, and the current PR is sufficient for most use cases.

Checklist:

• I have read the Contributing guidelines.