AuthN: Extract from OAuthServer service account management code (#76128)

* Extract code to manage service accounts

* Add test with client credentials grants

* Fix test with the changed interface

* Wire

* Fix HandleTokenRequest

* Add tests to extsvcaccounts

* Rename Retrieve function

* Document the interface

pkg/server/wire.go

@@ -64,6 +64,7 @@ import (
 	"github.com/grafana/grafana/pkg/services/encryption"
 	encryptionservice "github.com/grafana/grafana/pkg/services/encryption/service"
 	"github.com/grafana/grafana/pkg/services/extsvcauth"
+	"github.com/grafana/grafana/pkg/services/extsvcauth/extsvcaccounts"
 	"github.com/grafana/grafana/pkg/services/extsvcauth/oauthserver"
 	"github.com/grafana/grafana/pkg/services/extsvcauth/oauthserver/oasimpl"
 	extsvcreg "github.com/grafana/grafana/pkg/services/extsvcauth/registry"
@@ -359,6 +360,8 @@ var wireBasicSet = wire.NewSet(
 	authnimpl.ProvideIdentitySynchronizer,
 	authnimpl.ProvideAuthnService,
 	supportbundlesimpl.ProvideService,
+	extsvcaccounts.ProvideExtSvcAccountsService,
+	wire.Bind(new(extsvcauth.ExtSvcAccountsService), new(*extsvcaccounts.ExtSvcAccountsService)),
 	oasimpl.ProvideService,
 	wire.Bind(new(oauthserver.OAuth2Server), new(*oasimpl.OAuth2ServiceImpl)),
 	extsvcreg.ProvideExtSvcRegistry,