Building support for decoupling Dash from Flask and supporting Quart / FastAPI servers

[BSd3v]

This is an open PR draft, to contribute please target my forked branch.

The goal of this PR is to modularize the Dash setup to be independent of Flask (will fallback to Flask) and allow devs to configure their own backend.

• decouple callback context from Flask Request
• decouple index, assets, etc from build
• build Quart server support
• build FastAPI server support
• allow for custom server build
• add tests for servers

fixes #1571

[LiamConnors]

Does the server param docstring above need an update too?

Question - For the difference between the too - is it you'd use server if you want Dash to use an existing instance of the server? vs backend if you want Dash to create the instance?

[BSd3v]

Yes, I believe so.

You should be able to pass a preconfigured server of any of the supported types.

[T4rk1n]

@BSd3v This is looking good, I have made some significant changes to the fastapi implementation. The config is now transfered via environment variable and I merged all the middlewares into one.

Could you review those changes, then we can merge and release a rc version 🚀

[BSd3v]

@T4rk1n

I dont know that I necessarily agree with keeping the logic of set_header here, this appends to an already defined key, where as set makes the dev believe they are overriding. This function was done this way since headers.add in Flask is handled this way.

I think we should explicitly make it to where it is set, and if people want to append, then we can possibly expose a secondary headers append_header?