Bug 1059100 - [email] Implement XOAUTH2 authentication support for gmail. r=asuth, r=jrburke, r=mcav

landing gelam https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/336 too
Merge pull request #23800 from jrburke/bug1059100-email-oauth

Bug 1059100 - [email] Implement XOAUTH2 authentication support for gmail. r=asuth, r=jrburke, r=mcav