Add xoauth2 auth mechanism

[QHivert]

Add xoauth2 auth mechanism spec -> https://developers.google.com/workspace/gmail/imap/xoauth2-protocol
Supported by dovecot/pigeonhole

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.54%. Comparing base (842ba8d) to head (4b92c13).
⚠️ Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
sievelib/managesieve.py	75.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
- Coverage   87.60%   87.54%   -0.07%     
==========================================
  Files          10       10              
  Lines        2041     2055      +14     
==========================================
+ Hits         1788     1799      +11     
- Misses        253      256       +3     

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[tonioo]

@QHivert Thanks