feat(oauth): add flow for granting login by scanning a QR code

[Johennes]

This is a follow-up on #5801 and adds the opposite flow where the existing device scans the QR code generated on the new device.

I have tried to break this down into commits as far as seemed feasible. Unfortunately, git has made a bit off a mess out of the diff for the test cases in the last commit. It might be easier to review that part by looking at the whole file. The commit only adds entirely new test cases.

• Public API changes documented in changelogs (optional)

[codecov]

Codecov Report

❌ Patch coverage is 96.66667% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.60%. Comparing base (81ff96d) to head (488c3f5).
⚠️ Report is 7 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...atrix-sdk/src/authentication/oauth/qrcode/grant.rs	96.60%	14 Missing and 8 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5818      +/-   ##
==========================================
+ Coverage   88.54%   88.60%   +0.05%     
==========================================
  Files         361      361              
  Lines      101533   102140     +607     
  Branches   101533   102140     +607     
==========================================
+ Hits        89904    90502     +598     
- Misses       7418     7419       +1     
- Partials     4211     4219       +8     

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[Johennes]

@poljar I'm really sorry for the onslaught of tedious PRs around QR login. I think after this one there is, luckily, only one left for the bindings to grant login which should hopefully be small.

[codspeed-hq]

CodSpeed Performance Report

Merging #5818 will not alter performance

_{Comparing Johennes:johannes/qr-login-4 (488c3f5) with main (82c583b)}

Summary

✅ 50 untouched

[poljar]

Thanks, this looks mostly good. I found a couple of nits.

No worries about the amount of QR code PRs and sorry for the delay on this one.

[poljar]

Alright, looks good.

Let's just get rid of the fixup commits.

[Johennes]

History is clean now.