Add hooks for server customisation

[julianstirling]

In the OpenFlexure Microscope we need to customise the server (change some logging setup, mount some directories, etc). This means we can't use serve_from_cli, because we can't use serve_from_cli we need to reimplement the logic that brings up the fallback server.

It would be good to have a way to pass in a callable that modifies the sever between creation and being passed to Uvicorn.

Note that we also supply extra logging information to the fallback server, so we would need a hook for that too.

[rwb27]

I was wondering if it might make sense to move (some of) the functions from cli into the server class. That would make them easier to customise/override by subclassing the server.

That doesn't replace the need to structure it so there are good points to add customisation, but it might make it neater.

[rwb27]

#320 added a serve method to the server. This might be a good place to add the fallback option, which could then be more easily wrapped/customised in a subclass. Currently, the logic for the fallback server is still in cli.