add a way to manage generic webhook avatar & displayname

Nobody has claimed this yet.

with the old appservice you could set displayname and avatar through the webhook itself https://github.com/turt2live/matrix-appservice-webhooks#json-body-for-posting-messages
Actually, setting the displayname would also create the user in the first place. I'm not sure whether that last part is a good idea or not, however it allowed you to have multiple apps send their webhooks to the same appservice endpoint/token.

What I would like:
- edit displayname & avatar by the creator (or admins? or similar?) only?
- save them in room state probably, in which case admins can automatically edit it?
- edit by `!hookshot` command for convenience
- edit by integration manager UI for real convenience

Start by reviewing the generic webhook entry point, the !hookshot command, and the integration manager UI mentioned in the issue. Before implementation, clarify whether creators or admins may edit the avatar and display name, how values should be stored, and whether editing should create the user; done means the chosen flow works consistently with documented permissions.

Written by the indexing model from the issue text.