Add X-Grafana-Id header as const (#1045)

Author: jkroepke

backend/http_headers.go

@@ -16,6 +16,11 @@ const (
 	// OAuth Identity ID token.
 	OAuthIdentityIDTokenHeaderName = "X-Id-Token"
 
+	// GrafanaUserSignInTokenHeaderName the header name used for forwarding
+	// the SignIn token of a Grafana User.
+	// Requires idForwarded feature toggle enabled.
+	GrafanaUserSignInTokenHeaderName = "X-Grafana-Id" //nolint:gosec // this is not a hardcoded credential
+
 	// CookiesHeaderName the header name used for forwarding
 	// cookies.
 	CookiesHeaderName = "Cookie"