Add integration for FastAPI

[grahamalama]

Looking for some early feedback on this implementation of a Dockerflow integration with FastAPI.

Many more tests to write, but I wanted to get an early look at the general "API" of the integration (for now, documented only in how it's used in tests).

I'm also wondering if FastAPI is a library we want to actually support. It's a pretty popular library, but with all the talk at Mozilla about standardizing how we build products, it's worth considering if FastAPI is or should be one of those standard tools.

[jwhitlock]

I'm also wondering if FastAPI is a library we want to actually support. It's a pretty popular library, but with all the talk at Mozilla about standardizing how we build products, if FastAPI is or should be one of those standard tools.

I think if we can find a Mozilla employee that will use this integration, than we are justified to add it. If it won't be used by anyone, it is not worth adding.

I don't have any active FastAPI projects, so I'm not volunteering to sponsor. It has been a while since I've used FastAPI, but the code looks correct. I can probably dig in an give a full review later this week.

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (6c032ec) 98.23% compared to head (8ba84ef) 97.97%.

Files	Patch %	Lines
src/dockerflow/fastapi/middleware.py	93.02%	1 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #81 +/- ## ========================================== - Coverage 98.23% 97.97% -0.27% ========================================== Files 19 22 +3 Lines 624 692 +68 Branches 85 92 +7 ========================================== + Hits 613 678 +65 - Misses 7 8 +1 - Partials 4 6 +2 ```

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