AuthN: Set oauth client grant_types based on plugin state (#77248)

* Disable plugin service account

* Fix bug seen by linoman 💯

Co-authored-by: linoman <2051016+linoman@users.noreply.github.com>

* Account for PR feedback

Co-authored-by: linoman <2051016+linoman@users.noreply.github.com>

* Fix test data

* Enable datasource plugins by default

Co-authored-by: Andres Martinez Gotor <andres.martinez@grafana.com>

* Update pkg/services/extsvcauth/oauthserver/oasimpl/service.go

* Handle error differently

* Fix service reg

---------

Co-authored-by: linoman <2051016+linoman@users.noreply.github.com>
Co-authored-by: Andres Martinez Gotor <andres.martinez@grafana.com>

pkg/services/pluginsintegration/loader/loader_test.go

@@ -530,8 +530,7 @@ func TestLoader_Load_ExternalRegistration(t *testing.T) {
 				},
 				ExternalServiceRegistration: &plugindef.ExternalServiceRegistration{
 					Impersonation: &plugindef.Impersonation{
-						Enabled: boolPtr(true),
-						Groups:  boolPtr(true),
+						Groups: boolPtr(true),
 						Permissions: []plugindef.Permission{
 							{
 								Action: "read",