Add security customisation points

This adds various customisations point in the app for security related
decisions. By default, these do nothing, but would be customised at the
app level via module replacement (so that no changes are needed here in the
SDK).

Fixes https://github.com/vector-im/element-web/issues/15350

src/Login.ts

@@ -22,6 +22,7 @@ limitations under the License.
 import Matrix from "matrix-js-sdk";
 import { MatrixClient } from "matrix-js-sdk/src/client";
 import { IMatrixClientCreds } from "./MatrixClientPeg";
+import SecurityCustomisations from "./customisations/Security";
 
 interface ILoginOptions {
     defaultDeviceDisplayName?: string;
@@ -222,11 +223,15 @@ export async function sendLoginRequest(
         }
     }
 
-    return {
+    const creds: IMatrixClientCreds = {
         homeserverUrl: hsUrl,
         identityServerUrl: isUrl,
         userId: data.user_id,
         deviceId: data.device_id,
         accessToken: data.access_token,
     };
+
+    SecurityCustomisations.examineLoginResponse?.(data, creds);
+
+    return creds;
 }