Split off from https://github.com/matrix-org/synapse/pull/9491

Adds a storage method for getting the current presence of all local users, optionally excluding those that are offline. This will be used by the code in #9491 when a PresenceRouter module informs Synapse that a given user should have `"ALL"` user presence updates routed to them. Specifically, it is used here: https://github.com/matrix-org/synapse/blob/b588f16e391d664b11f43257eabf70663f0c6d59/synapse/handlers/presence.py#L1131-L1133

Note that there is a `get_all_presence_updates` function just above. That function is intended to walk up the table through stream IDs, and is primarily used by the presence replication stream. I could possibly make use of it in the PresenceRouter-related code, but it would be a bit of a bodge.