feat: Add AuthMechanism::XOAUTH2.

[coveralls]

Pull Request Test Coverage Report for Build 5359637409

• 98 of 103 (95.15%) changed or added relevant lines in 11 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.2%) to 93.29%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
imap-types/src/extensions/quota.rs	15	16	93.75%
imap-types/src/auth.rs	12	14	85.71%
src/extensions/idle.rs	0	2	0.0%
<!--	Total:	98	103	95.15%	-->

Files with Coverage Reduction	New Missed Lines	%
imap-types/src/extensions/quota.rs	1	91.86%
src/extensions/idle.rs	1	93.88%
<!--	Total:	2		-->

Totals
Change from base Build 5359249286:	0.2%
Covered Lines:	9065
Relevant Lines:	9717

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💛 - Coveralls