feat: add tooltip support for PDF files

[leon-richardt]

Pull request checklist:

• CHANGELOG.md was updated, if applicable

Description

Closes #363.

This PR adds the concept of ContentTypeResolvers. With these, link resolving can be specialized by the response's Content-Type header value rather than by matching on the URL.

This concept is then implemented for the content type application/pdf. For links with this content type, a tooltip with the following information is generated:

• Title
• Author
• Number of pages
• Creation date
• Thumbnail of first page

Review Notes

Points a reviewer can focus on:

1. Do you agree with the way ContentTypeResolvers are created and passed to the LinkLoader?
2. I decided to put the ContentTypeResolver interface and the PDFResolver implementation in the defaultresolver package. Would you rather have them elsewhere or in a subpackage?
3. Do you agree with the format of the PDF tooltip? Should there be more fields?
4. Currently, the export format of thumbnails for PDF files is hardcoded. Should there be a more general way of defining these?

Testing

Links to test this with:

• https://github.com/leon-richardt/green-networking/raw/194cfcc20bb059c62f5b305b1a58f17c28d8c1c4/Presentation/server_consolidation.pdf
  • Won't resolve correctly since GitHub returns application/octet-stream instead of application/pdf
• https://jaf.leon.dev/ZphEa.pdf
  • Same document as above but with correct Content-Type header
• https://www.cs.virginia.edu/~robins/Turing_Paper_1936.pdf

---

Thank you!

[codecov]

Codecov Report

Merging #374 (fb185bd) into master (dbc34af) will decrease coverage by 0.63%.
The diff coverage is 21.59%.

@@            Coverage Diff             @@
##           master     #374      +/-   ##
==========================================
- Coverage   45.97%   45.34%   -0.64%     
==========================================
  Files          99      100       +1     
  Lines        3589     3670      +81     
==========================================
+ Hits         1650     1664      +14     
- Misses       1893     1959      +66     
- Partials       46       47       +1     

Impacted Files	Coverage Δ
internal/resolvers/default/model.go	100.00% <ø> (ø)
pkg/thumbnail/thumbnail.go	0.00% <0.00%> (ø)
internal/resolvers/default/pdf_resolver.go	12.96% <12.96%> (ø)
internal/resolvers/default/link_loader.go	49.50% <25.00%> (-6.45%)	⬇️
internal/resolvers/default/link_resolver.go	42.92% <100.00%> (+1.17%)	⬆️

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