Request response storage typing

[gsoldatov]

What do these changes do?

• added RequestKey & ResponseKey classes, which enable static typing checks in corresponding classes with dict-like storages, similarly to AppKey;
• moved common logic of key classes into a super-class;
• added tests for new classes & updated docs;
• fixed a few flake8 errors.

Are there changes in behavior for the user?

Users may now use RequestKey & ResponseKey in request & response objects, in addition to string keys.

Is it a substantial burden for the maintainers to support this?

Related issue number

This issue should close #11762.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
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[codspeed-hq]

CodSpeed Performance Report

Merging #11766 will not alter performance

_{Comparing gsoldatov:request_response_storage_typing (66aae89) with master (056d929)}

Summary

✅ 59 untouched