Ability to delegate user search API through to 3rd party directory services

[ara4n]

element-hq/element-web#2930 covered off the need for an API and UX for serverside user search in general. However, it only implemented the 'mastodon semantics' solution - i.e. where the HS searches only the set of users it knows about which you are sharing a room with (or are in a public room).

It would be very nice to have a way to:

• delegate through to 3rd party directory services (LDAP, Active Directory, CardDAV etc)
• search some kind of global Matrix user directory in future (perhaps integrated with the Identity Service).

[maxidorius]

mxisd approach to handle this in a user friendly way.

[hex-m]

ma1sd has that functionality but it would be great to have it in synapse. This issue probably doesn't belong into the element-web project except if we want to search in a client-side backend like Gnome Contacts.