feat: Add support for HPKE

[poljar]

This replaces #289.

Cleans up the history a bit and rebases on top of main now that #206 is merged, otherwise it's the same as #289.

I left the branch for #289 alive as the Rust SDK PR uses the git revisions and I didn't want to break the build as I

[codspeed-hq]

Merging this PR will not alter performance

✅ 6 untouched benchmarks

---

_{Comparing poljar/hpke-pr (fd6770f) with main (6b38b2c)}

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.56631% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.69%. Comparing base (6b38b2c) to head (fd6770f).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/hpke/messages.rs	95.94%	0 Missing and 3 partials ⚠️
src/hpke/sender.rs	96.62%	0 Missing and 3 partials ⚠️
src/hpke/mod.rs	99.45%	1 Missing ⚠️
src/hpke/recipient.rs	99.11%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #377      +/-   ##
==========================================
+ Coverage   90.93%   91.69%   +0.75%     
==========================================
  Files          34       40       +6     
  Lines        4955     5501     +546     
  Branches     4955     5501     +546     
==========================================
+ Hits         4506     5044     +538     
- Misses        286      287       +1     
- Partials      163      170       +7     

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