Add FastAPI based serve functions

[philippjfr]

Builds on top of bokeh-fastapi and adds two main user facing APIs:

• panel.io.fastapi.serve: Mirrors the panel.serve API but launches a FastAPI backed uvicorn server (optionally on a separate thread).
• panel.io.fastapi.add_applications: Adds Panel applications to an existing FastAPI application.
• panel.io.fastapi.add_application: Decorator to add a function returning Panel objects to the FastAPI server.

@pmeier, thoughts and review appreciated. I'd like to get an initial cut of this in pretty soon but then we'll have about 1 - 1.5 weeks to add docs and more tests.

[codecov]

Codecov Report

Attention: Patch coverage is 78.06452% with 68 lines in your changes missing coverage. Please review.

Project coverage is 82.23%. Comparing base (647db1a) to head (121c109).
Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/io/fastapi.py	63.15%	35 Missing ⚠️
panel/io/server.py	43.47%	13 Missing ⚠️
panel/io/application.py	83.07%	11 Missing ⚠️
panel/io/threads.py	85.71%	7 Missing ⚠️
panel/tests/test_server.py	96.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7205      +/-   ##
==========================================
- Coverage   82.24%   82.23%   -0.02%     
==========================================
  Files         331      337       +6     
  Lines       49420    50123     +703     
==========================================
+ Hits        40647    41217     +570     
- Misses       8773     8906     +133     

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[MarcSkovMadsen]

If this make it very easy to add FAST API (REST) endpoints in general it would be of great value.

[philippjfr]

This pull request has been mentioned on HoloViz Discourse. There might be relevant details there:

https://discourse.holoviz.org/t/how-to-integrate-a-panel-app-with-fastapi-on-the-same-port/7601/6