message: Fix secret sharing names to match spec
  When sharing keys, we should use `m.cross_signing` prefix.
  Part of
diff: @@ -144,10 +144,10 @@ const onSecretRequested = async function({
  }
  const callbacks = client.getCrossSigningCacheCallbacks();
  if (!callbacks.getCrossSigningKeyCache) return;
  - if (name === "m.key.self_signing") {
  + if (name === "m.cross_signing.self_signing") {
  const key = await callbacks.getCrossSigningKeyCache("self_signing");
  return key && encodeBase64(key);
  - } else if (name === "m.key.user_signing") {
  + } else if (name === "m.cross_signing.user_signing") {
  const key = await callbacks.getCrossSigningKeyCache("user_signing");
  return key && encodeBase64(key);
  }
  