Invitation system rework

[vxgmichel]

There's been an ongoing effort to change the invitation system to decouple the creator of an invitation from the possible greeters for this same invitation.

This is obviously the case for shamir invitations, and the implementation of the non-blocking invite transport took this into account in order to generalize the fact that a claimer starts a greeting attempt with the greeter that they chose from the information provided in invitation_info.

As explained in #9078, this opens the way to support multiple greeters (i.e the organization administrators) for a user invitation. Even better, the invitation itself could be created by an external service such as Parsec Auth or an LDAP.

Since the new API v5 is being released in parsec 3.3, we want to make sure that issues #9078, #9410 and #9411 are included in the release.

This way, issues #9412 and #9413 can then be implemented later once their priority is high enough. Still, a proof of concept for #9413 could be useful in order to validate the work done in previous PRs before version 3.3 is relased with API v5.

This meta-issue tracks all issues that relates to the overall implementation of this new approach to invitations and will be deemed completed once we have:

• the possibility for an external service to create an invitation
• the possibility for a new user to be greeted by any administrator
• notifications for the administrator to know when a new user is waiting for an identity validation

[mmmarcos]

Discussed today: issues #9412 #9413 #9415 can be done after the v3.3 release. Even if @vxgmichel would like to at least try a POC before.