Add a convenience method to `Room` for getting member IDs without the functional members

[pixlwave]

Complementary to #6490, now that the SDK can do DM computation, it would be ideal for the SDK to also compute who is the opposite party in a DM.

Whilst a fn Room::dm_recipient() -> String? could potentially be used (retuning None when not a DM), the more general solution here would be something like fn Room::human_members() -> Vec<String> which returns an array of members who aren't functional members.