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

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

pkg/plugins/plugindef/plugindef.cue

@@ -425,9 +425,6 @@ schemas: [{
 		}
 
 		#Impersonation: {
-			// Enabled allows the service to request access tokens to impersonate users using the jwtbearer grant
-			// Defaults to true.
-			enabled?: bool
 			// Groups allows the service to list the impersonated user's teams.
 			// Defaults to true.
 			groups?: bool