Adds missing SAMLAuthProvider constructor. (#2513)

Fixes firebase/firebaseui-web#663

Author: bojeil-google

packages/auth-types/index.d.ts

@@ -223,6 +223,7 @@ export class OAuthProvider implements AuthProvider {
 }
 
 export class SAMLAuthProvider implements AuthProvider {
+  constructor(providerId: string);
   providerId: string;
 }
 

packages/firebase/index.d.ts

@@ -3662,6 +3662,7 @@ declare namespace firebase.auth {
   }
 
   class SAMLAuthProvider implements firebase.auth.AuthProvider {
+    constructor(providerId: string);
     providerId: string;
   }