fix: handle incoming requests on the api level

[thomasheartman]

Update all endpoints of the frontend_api to accept top-level properties.

This is primarily just changing the type we expect and then, on the lower level functions, converting the incoming context into a regular context.

Most of the functions are pretty straight-forward. get_all_features appears to be the only one that doesn't share an implementation with any other functions.

The tests check:

• /api/frontend/features/:feature (/api/proxy/features/:feature uses the same impl under the hood, so I've not tested that explicitly. Happy to change that though.
• GET /api/{frontend,proxy}(/all)?
• POST /api/{frontend,proxy) (POST all features function takes an IncomingContext as a parameter, so I'm assuming the serialization here works; can of course add tests for that too)
• That providing a string as the value of properties yields a 400

Are there any more endpoints to update? Any other input?

[thomasheartman]

Ah, there is one edge case that I can think of, actually: Deserializing empty string properties.

I noticed that the base Context type is defined with a custom deserializer:

        deserialize_with = "remove_null_properties",

With the new extra_properties, these can be empty strings. Should they be stripped out in any way when deserializing?

(Also: added a test for the invalid properties value giving 400 as mentioned in the previous PR)