IMAP: add support for native GMail OAuth authentication

This change adds support for XOAUTH2 authentication method used by
Gmail to the IMAP resource. The XOAUTH2 support is implemented in
a custom SASL plugin. The token request/refresh is done via KGAPI.

When user sets imap.gmail.com as IMAP server in the resource config
dialog, the dialog automatically sets all the configuration to match
the one of Gmail and selects XOAUTH2 as authentication method. The
access and refresh tokens are stored in KWallet like a regular
password, but a special PasswordRequesterInterface implementation is
used to handle the tokens and pass the right data to KIMAP::LoginJob.

With this change it's no longer necessary to have the "Allow less
secure apps" option enabled in Google Account settings and it's no
longer necessary to use app-specific password for accounts with
two-step verification (2FA) enabled. The actual password is no longer
stored in KWallet and has only be typed into Google Auth form once.

FEATURE: 354508
FIXED-IN: 5.5.0
CHANGELOG: Implement native Gmail authentication into IMAP resource