Add OAuth support to KSmtp. We already have the SASL plugin in kdepim-runtime and KSmtp has SASL support, so it's just about adding new values to the mech enum and adjusting UI in MailTransport. The SASL plugin could probably be moved to libkgapi, which is a hard dependency of PIM now anyway and is a good central place for this.

The trick is to try to share the OAuth token with the IMAP resource, because I'm not sure if otherwise there won't be problem with IMAP and KSmtp fighting over a valid token for the same app keys. Again, I wonder if I should add some token store/handler to libkgapi to make that easier or if we should start looking into having some centralized account management or token provider for whole KDE PIM (backed by kwallet or whatever) so that stuff like OAuth tokens are not bound to individual apps or resource instances.