feat(oauth): add flow for reciprocating a login using a QR code generated on the existing device

[Johennes]

This adds the flow for reciprocating a login by generating a QR code on the logged in device as per MSC4108. Two required refactoring commits are also included which I hope is ok as they felt too small to justify being put into their own PR. I'm planning to add the complementary flow where the existing device scans the QR code in a separate PR.

• Public API changes documented in changelogs (optional)

[codspeed-hq]

CodSpeed Performance Report

Merging #5801 will not alter performance

_{Comparing Johennes:johannes/qr-login-3 (3190a35) with main (34c5e24)}

Summary

✅ 50 untouched

[codecov]

Codecov Report

❌ Patch coverage is 91.14391% with 48 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.47%. Comparing base (430304f) to head (3190a35).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...src/authentication/oauth/qrcode/reciprocate/mod.rs	68.60%	18 Missing and 9 partials ⚠️
...authentication/oauth/qrcode/reciprocate/scanned.rs	95.17%	13 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5801      +/-   ##
==========================================
+ Coverage   88.45%   88.47%   +0.01%     
==========================================
  Files         360      362       +2     
  Lines      100328   100865     +537     
  Branches   100328   100865     +537     
==========================================
+ Hits        88749    89239     +490     
- Misses       7413     7443      +30     
- Partials     4166     4183      +17     

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