Support for MSC4388 for the QR login

[poljar]

• Public API changes documented in changelogs (optional)

[codecov]

Codecov Report

❌ Patch coverage is 95.61243% with 48 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.90%. Comparing base (7beda79) to head (cb359ee).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...cation/oauth/qrcode/rendezvous_channel/msc_4388.rs	95.79%	11 Missing and 2 partials ⚠️
...atrix-sdk/src/authentication/oauth/qrcode/login.rs	95.77%	10 Missing and 2 partials ⚠️
...tion/oauth/qrcode/secure_channel/crypto_channel.rs	67.85%	0 Missing and 9 partials ⚠️
.../authentication/oauth/qrcode/secure_channel/mod.rs	94.70%	4 Missing and 4 partials ⚠️
...hentication/oauth/qrcode/rendezvous_channel/mod.rs	89.47%	0 Missing and 4 partials ⚠️
...atrix-sdk/src/authentication/oauth/qrcode/grant.rs	98.55%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6173      +/-   ##
==========================================
+ Coverage   89.81%   89.90%   +0.09%     
==========================================
  Files         376      377       +1     
  Lines      102614   103507     +893     
  Branches   102614   103507     +893     
==========================================
+ Hits        92163    93063     +900     
+ Misses       6871     6854      -17     
- Partials     3580     3590      +10     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

---

_{Comparing poljar/qr-login/msc4388 (cb359ee) with main (7beda79)}

[hughns]

// n.b. this doesn't match the MSC which says that it is a server name not the base URL
// Should be something like self.client.user_id_server_name()