Adding OAuth2 authentication support

Implements two functions:
`mailimap_oauth2_authenticate` Use to to send the authentication token to the server and actually authenticate with it.
`mailimap_oauth2_authenticate_send` Builds and sends the authentication string in the form of `base64("user=" {User} "^Aauth=Bearer " {Access Token} "^A^A")``
This has been tested against Gmail.