ACL settings editing for shared mailboxes

[miaulalala]

Is your feature request related to a problem? Please describe.

If an IMAP server supports ACLs, there might be users who have permission a on a shared mailbox, or users want to share a mailbox with other users on the same IMAP server.

Describe the solution you'd like

Create an API that allows adding and removing ACL permissions on mailboxes and create a frontend dialogue to modify these ACL permissions on a per- mailbox basis.

Describe alternatives you've considered

We could also limit these options to users in the admin group and allow these settings to be modified via occ commands - althougt there is a case to be made that the primary control for ACLs being available to a user should be on the IMAP server itself as global ACLs, and should not be in the purview of the Mail app.

Additional context

https://github.com/nextcloud/mail/issues/7057