Centralize check registration / running logic

[grahamalama]

This PR adds to the dockerflow.checks package to provide the ability to register checks, run registered checks (both synchronously and asynchronously), and return the results of the checks in a standardized manner.

Since Django already has the concept of a check registry, we just run the checks there (but with our check runner). For other frameworks, we use the new registry in dockerflow.checks.registry.

Still todo is ~add tests for the check running functions~ (to make assertions about the shape of the data they return) and update documentation. But I'm open to review in this PR's current state.

[codecov-commenter]

Codecov Report

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

Comparison is base (a7cad34) 97.84% compared to head (880630f) 98.23%.

Files	Patch %	Lines
src/dockerflow/checks/registry.py	96.29%	1 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #85 +/- ## ========================================== + Coverage 97.84% 98.23% +0.39% ========================================== Files 18 19 +1 Lines 602 624 +22 Branches 88 85 -3 ========================================== + Hits 589 613 +24 + Misses 9 7 -2 Partials 4 4 ```

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[leplatrem]

@grahamalama shall we resume this work?

[leplatrem]

@grahamalama r?

[grahamalama]

Nice @leplatrem, thanks for taking this over and finishing it 🙌🏻

But oh no, I can't actually approve this PR because I was an author on some of the commits 😱.

You could pull another reviewer in, or you can consider this my approval and bypass branch protections.