Invoke Secure Backup flow inside the app when requested by HS

If the Secure Backup required mode is set the client `.well-known` file, then
this will ensure that everyone already inside the app is required to complete
setup matching that policy.

Fixes https://github.com/vector-im/element-web/issues/14954

src/utils/WellKnownUtils.ts

@@ -23,6 +23,14 @@ export interface IE2EEWellKnown {
     default?: boolean;
 }
 
+export async function ensureClientWellKnown() {
+    const cli = MatrixClientPeg.get();
+    if (cli.haveAttemptedFetchingClientWellKnown()) return;
+    return new Promise(resolve => {
+        cli.once("WellKnown.attempted", resolve);
+    });
+}
+
 export function getE2EEWellKnown(): IE2EEWellKnown {
     const clientWellKnown = MatrixClientPeg.get().getClientWellKnown();
     if (clientWellKnown && clientWellKnown[E2EE_WK_KEY]) {