Add FastAPI API property to Air application

[pydanny]

Issue(s)

#575

Description

Introduces an optional FastAPI app mounted at '/api' via the new 'fastapi_app' parameter and 'api' property in the Air class. Adds tests to verify API route mounting and response behavior.

This will make it easier for project creators to combine Air and FastAPI, since they are already one project. The api property can be prevented.

Currently state, including lifespan, isn't shared. For reference, it is not easy for this feature to share state.

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• New feature
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

Pull request tasks

The following have been completed for this task:

• [c] Code changes
• Documentation changes for new or changed features
• Alterations of behavior come with a working implementation in the examples folder
• Tests on new or altered behaviors

Demo or screenshot

import air

app = Air()

@app.page
def index() -> air.H1:
    return air.H1("Hello, World!")


@app.api.get("/items")
def get_items():
    return {"items": ["one", "two", "three"]}

Checklist

• I have run just test and just qa, ensuring my code changes passes all existing tests
• I have performed a self-review of my own code
• I have ensured that there are tests to cover my changes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[pygarap]

Should be: FastAPI()

[pygarap]

Should be: = fastapi_app

[pygarap]

"/app" should be customizable.

[pygarap]

Rename to: api_sub_app