feat: add app started callback

[MaikoTan]

This is useful like when you want to add a custom API route from extensions or do some post-processing for FastAPI class.

[Interpause]

If mounting custom APIs, what would the recommended path be?

I am currently thinking /sdapi/ext/<repo_name>

[MaikoTan]

If mounting custom APIs, what would the recommended path be?

I am currently thinking /sdapi/ext/<repo_name>

I think you can use any custom paths that matches your own, as long as not to override existing ones.
For example, I have an app named dream, so I registered my custom API like this:

from fastapi import FastAPI
import gradio as gr

import modules.script_callbacks as script_callbacks

def dream(demo: gr.Blocks, app: FastAPI):
    @app.get("/dream/start")
    async def start_dream():
        # ...

    @app.get("/dream/stop")
    async def stop_dream():
        # ...

script_callbacks.on_app_started(dream)