Experiment: Webxdc: add virtual directory for fetching avatars and group memberlist

[Simon-Laux]

There are still many open questions, this is just a prototype, see the comments in the code. The most controversial thing is the member list, I needed it to make the avatars work, so I already exposed it. But we can also postpone the memberlist webxdc api and just do the avatar api for now.

The changes:

• adds a webxdc.getMemberList which returns the group members for which we have public keys (because the webxdc user id is based on public key + webxdc message rfc id)
  • The format is [userid:string, displayName:string][]
• adds a way to get user avatars over a virtual directory: __webxdc__/avatar/<user_id>.jpg
  • if the user has no profile picture this returns 404, we could think about returning a color image or text avatar, but we don't want to deal with text rendering in core, so I think it is fine like it is.B

user_id is basically the (self)addr

Desktop pr so you can test the changes - you can test avatars without it, only needed for memberlist api: (https://github.com/deltachat/deltachat-desktop/pull/4481)