code: function encryptGroupSessionKey(opts) {
      return encryptOlmEvent({
          senderKey: opts.senderKey,
          recipient: opts.recipient,
          p2pSession: opts.p2pSession,
          plaincontent: {
              algorithm: 'm.megolm.v1.aes-sha2',
              room_id: opts.room_id,
              session_id: opts.groupSession.session_id(),
              session_key: opts.groupSession.session_key(),
          },
          plaintype: 'm.room_key',
      });
  }
docs: build an encrypted room_key event to share a group session
  
  @param {object} opts
  @param {string} opts.senderKey
  @param {TestClient} opts.recipient
  @param {Olm.Session} opts.p2pSession
  @param {Olm.OutboundGroupSession} opts.groupSession
  @param {string=} opts.room_id
  
  @return {object} event