Support to log in using a QR code

[poljar]

This implements part of MSC4108. We're only supporting the case where new device scans the QR code and gets logged in by the existing device.

Opening as a draft as I want to reword the commit messages a bit, add some more documentation (mainly to insert some jokes about rendezvous), and possibly one or two more tests.

Best reviewed commit by commit, although there is one giant commit. Reviewer please have mercy on me.

• Public API changes documented in changelogs (optional)

[Hywan]

Looking really good! Congrats for the quality of the code and the commits, thank you.

I've left a couple of feedback, but nothing too difficult to fix.

[Hywan]

Thanks for addressing my feedback!

[poljar]

Thanks for addressing my feedback!

No no, thanks for the swift review.

[codecov]

Codecov Report

Attention: Patch coverage is 93.53846% with 21 lines in your changes are missing coverage. Please review.

Project coverage is 83.65%. Comparing base (664a64b) to head (4d8bfbd).

Files	Patch %	Lines
...ates/matrix-sdk/src/authentication/qrcode/login.rs	89.74%	12 Missing ⚠️
...dk/src/authentication/qrcode/rendezvous_channel.rs	95.06%	4 Missing ⚠️
...ix-sdk/src/authentication/qrcode/secure_channel.rs	92.10%	3 Missing ⚠️
...atrix-sdk/src/authentication/qrcode/oidc_client.rs	96.87%	1 Missing ⚠️
crates/matrix-sdk/src/oidc/mod.rs	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3466      +/-   ##
==========================================
+ Coverage   83.28%   83.65%   +0.36%     
==========================================
  Files         248      254       +6     
  Lines       25272    25592     +320     
==========================================
+ Hits        21049    21408     +359     
+ Misses       4223     4184      -39     

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