Add ability to get provider by name from auth service

src/auth/Authentication.ts

@@ -27,6 +27,15 @@ export class Authentication extends Unit {
 
     protected providers: {[name: string]: LoginProvider<LoginProviderConfig>} = {}
 
+    getProvider(name: string): LoginProvider<LoginProviderConfig> {
+        const provider = this.providers[name]
+        if ( !provider ) {
+            throw new ErrorWithContext('Invalid auth provider name: ' + name, { name })
+        }
+
+        return provider
+    }
+
     async up(): Promise<void> {
         this.middleware.registerNamespace('@auth', this.getMiddlewareResolver())