Global guest name setting

[juliusknorr]

Since more and more apps allow users without an account to set a guest name if they use a public sharing link, we should provide a server api to properly store the name so that all apps (talk, text, onlyoffice, collabora) can use it.

For live updating open sessions with the new name we could use the local storage onChanged event.

For the backend I would propose to have a similar interface to IUserSession e.b. IGuestSession. It might make sense to have a dedicated unique id in case apps store additional information for the guest user, so a user name change will not lead to issues.

cc @nickvergessen

@jancborchardt Maybe we should also discuss some more unified way of setting the guest user? Super quick mockup just for the positioning:

[juliusknorr]

Maybe it should also handle guest avatars as in nextcloud/spreed#357

[nickvergessen]

Yeah, as long as updating the name goes to a central location and triggers an event as well, so apps like talk can listen to it and update the database it sounds good to me.

[joshtrichards]

Related: nextcloud-libraries/nextcloud-browser-storage#3