feat(webdav): Add configurable extra footer HTML support

[SilviaSWR]

Description

This PR adds support for injecting a custom footer into the generated HTML via extra_footer_path.

The footer content is loaded from a file and inserted into the HTML template, allowing configurable footer content in directory listings.

No changes are made to static file serving or resource handling.

fixes #91

Checklist

• I have read the contribution guidelines
• One feature (issue) per PR
• All commits will be squashed into a single commit
• Changelog updated if necessary
• Tests added/updated if necessary
• Documentation updated if necessary

[SilviaSWR]

This contribution has received funding from the Spanish government (grant EQC2021-007479-P, funded by MCIN/AEI/10.13039/501100011033), the EU NextGeneration/PRTR (PRTR-C17.I1), and the Generalitat de Catalunya.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.91%. Comparing base (412893d) to head (695f6c2).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
asgi_webdav/web_dav.py	45.45%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #92      +/-   ##
==========================================
- Coverage   76.00%   75.91%   -0.09%     
==========================================
  Files          26       26              
  Lines        3775     3787      +12     
==========================================
+ Hits         2869     2875       +6     
- Misses        906      912       +6     

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