feat(static): implement Last-Modified header for static route

[Cycloctane]

Summary of Changes

• Render Last-Modified header for static file responses.
• Check request If-Modified-Since header and return 304 if date in this header is not earlier than mtime.
• Return 403 Forbidden for PermissionError when serving static.

Related Issues

Fixes #2244

Pull Request Checklist

• Applied changes to both WSGI and ASGI code paths and interfaces (where applicable).
• Added tests for changed code.
• Prefixed code comments with GitHub nick and an appropriate prefix.
• Coding style is consistent with the rest of the framework.
• Updated documentation for changed code.
  • Added docstrings for any new classes, functions, or modules.
  • Updated docstrings for any modifications to existing code.
  • Updated both WSGI and ASGI docs (where applicable).
  • Added references to new classes, functions, or modules to the relevant RST file under docs/.
  • Updated all relevant supporting documentation files under docs/.
  • A copyright notice is included at the top of any new modules (using your own name or the name of your organization).
  • Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives.
• Changes (and possible deprecations) have towncrier news fragments under docs/_newsfragments/, with the file name format {issue_number}.{fragment_type}.rst. (Run towncrier --draft to ensure it renders correctly.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (8e866bd) to head (c3f1d58).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2426   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           64        64           
  Lines         7729      7748   +19     
  Branches      1071      1073    +2     
=========================================
+ Hits          7729      7748   +19     

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[CaselIT]

nice improvement.

Some lines are not covered by tests, if you could add tests for them it would be great