Ability to set room topic and avatar via generic webhooks

This could for instance be used to reflect the state of something in the room avatar (red failed; green ok) and the topic.

I could imagine at least two ways of doing this:

1. Extend generic webhooks to send other events than m.room.message.
   It should also support sending multiple messages per call:
   webhook → room message + update room avatar/topic
   Multiple messages could be achieved by turning result into an array.

2. Add a roomTopic and roomAvatar (mxc) prop to result.
   Generic webhook handler then sends the state events in addition to the message.

Start by tracing the generic webhook handler and the current result shape. Compare the two proposed approaches for sending room messages alongside room topic and avatar state events, including the suggested array form for multiple messages. Done means the issue's chosen design is implemented and the webhook behavior is covered for topic, avatar, and message updates.