asgi_wrapper plugin hook

[simonw]

After #272 we can finally add this hook. It will allow plugins to wrap their own ASGI middleware around Datasette. Potential use-cases include:

• adding authentication
• custom CORS headers (see Plugin for allowing CORS from specified hosts #454)
• maybe gzip support?
• possibly defining entirely new routes, though that may be better handled by a separate hook

[simonw]

These plugins will need access to configuration and the ability to execute SQL - which means we need to make the datasette instance available to them.

[simonw]

#272 is closed now! This hook is next on the priority list.

[simonw]

Documentation for the new hook: https://datasette.readthedocs.io/en/latest/plugins.html#asgi-wrapper-datasette