message: use the default SSSS key if the default is set
  implements MSC2874
diff: @@ -98,11 +98,24 @@ async function getSecretStorageKey(
  { keys: keyInfos }: { keys: Record<string, ISecretStorageKeyInfo> },
  ssssItemName,
  ): Promise<[string, Uint8Array]> {
  + const cli = MatrixClientPeg.get();
  + let keyId = await cli.getDefaultSecretStorageKeyId();
  + let keyInfo;
  + if (keyId) {
  + // use the default SSSS key if set
  + keyInfo = keyInfos[keyId];
  + if (!keyInfo) {
  + throw new Error("Unable to use default SSSS key");
  + }
  + } else {
  + // if no default SSSS key is set, fall back to a heuristic of using the
  + // only available key, if only one key is set
  const keyInfoEntries = Object.entries(keyInfos);
  if (keyInfoEntries.length > 1) {
  throw new Error("Multiple storage key requests not implemented");
  }
  - const [keyId, keyInfo] = keyInfoEntries[0];
  + [keyId, keyInfo] = keyInfoEntries[0];
  + }
  // Check the in-memory cache
  if (isCachingAllowed() && secretStorageKeys[keyId]) {
  