Add setGithubButtonId

Author: Bernat Borrás Paronella

auth/src/main/java/com/firebase/ui/auth/AuthMethodPickerLayout.java

@@ -135,6 +135,14 @@ public AuthMethodPickerLayout.Builder setEmailButtonId(@IdRes int emailButton) {
             return this;
         }
 
+        /**
+         * Set the ID of the Github sign in button in the custom layout.
+         */
+        public AuthMethodPickerLayout.Builder setGithubButtonId(@IdRes int githubButton) {
+            providersMapping.put(GithubAuthProvider.PROVIDER_ID, githubButton);
+            return this;
+        }
+
         /**
          * Set the ID of the Phone Number sign in button in the custom layout.
          */