IDForwarding: Add service and a local signer (#75423)

* IDForwarding: Add service for handling id token and create a local signer
---------

Co-authored-by: Ieva <ieva.vasiljeva@grafana.com>

pkg/services/auth/id.go

@@ -0,0 +1,21 @@
+package auth
+
+import (
+	"context"
+
+	"github.com/go-jose/go-jose/v3/jwt"
+	"github.com/grafana/grafana/pkg/services/auth/identity"
+)
+
+type IDService interface {
+	// SignIdentity signs a id token for provided identity that can be forwarded to plugins and external services
+	SignIdentity(ctx context.Context, identity identity.Requester) (string, error)
+}
+
+type IDSigner interface {
+	SignIDToken(ctx context.Context, claims *IDClaims) (string, error)
+}
+
+type IDClaims struct {
+	jwt.Claims
+}