Add metadata for videoroom publishers

[spscream]

I want to introduce ability to set metadata for publishers on join.
I added metadata field with json_t and save info to it on join and send it in "joined"/"published" events.

We use it on our branch now to build custom business logic for videoroom, for example we set in metadata now:

• server_id from which user has connected(we build remote publishers logic for our backends around this field)
• users ip/network, we use it to build acl for publishers, we have "contours" feature for our publishers, screen sharing of user can be viewed only from users from the same network/contour/group/role as publisher

[danjenkins]

I've used display with stringified json before for exactly this purpose so it would be great to have a proper meta data field

[lminiero]

It does make sense, yes, but as it is it looks like it's an immutable object. I'd add an option to modify the metadata object in a configure request (and notify the change to other attendees), just as you can edit the display name dynamically.

[spscream]

added ability to update metadata

[lminiero]

Thanks for the changes, looks good to me! Once you fix the conflicts, I think this is ready to be merged.

[spscream]

@lminiero rebased

[lminiero]

Thanks! The only thing I'm wondering about, now, is that if you use a configure to update both display and metadata (and maybe something else) you get different notifications for eadh of them, instead of a single event that notifies a change on everything. But that's something I can look into later on, especially considering it may have been an issue before already (e.g., with display and other properties).