allow synchronizing user status from OAuth2 login providers (#31572)

This leverages the existing `sync_external_users` cron job to
synchronize the `IsActive` flag on users who use an OAuth2 provider set
to synchronize. This synchronization is done by checking for expired
access tokens, and using the stored refresh token to request a new
access token. If the response back from the OAuth2 provider is the
`invalid_grant` error code, the user is marked as inactive. However, the
user is able to reactivate their account by logging in the web browser
through their OAuth2 flow.

Also changed to support this is that a linked `ExternalLoginUser` is
always created upon a login or signup via OAuth2.

### Notes on updating permissions
Ideally, we would also refresh permissions from the configured OAuth
provider (e.g., admin, restricted and group mappings) to match the
implementation of LDAP. However, the OAuth library used for this `goth`,
doesn't seem to support issuing a session via refresh tokens. The
interface provides a [`RefreshToken`
method](https://github.com/markbates/goth/blob/master/provider.go#L20),
but the returned `oauth.Token` doesn't implement the `goth.Session` we
would need to call `FetchUser`. Due to specific implementations, we
would need to build a compatibility function for every provider, since
they cast to concrete types (e.g.
[Azure](https://github.com/markbates/goth/blob/master/providers/azureadv2/azureadv2.go#L132))

---------

Co-authored-by: Kyle D <kdumontnu@gmail.com>

models/auth/source.go

@@ -210,7 +210,7 @@ func CreateSource(ctx context.Context, source *Source) error {
 		return ErrSourceAlreadyExist{source.Name}
 	}
 	// Synchronization is only available with LDAP for now
-	if !source.IsLDAP() {
+	if !source.IsLDAP() && !source.IsOAuth2() {
 		source.IsSyncEnabled = false
 	}