Introduce support for MSC4429: Profile Updates for Legacy Sync (#19556)

Implements support for [MSC4429: Profile Updates for Legacy
Sync](https://github.com/matrix-org/matrix-spec-proposals/pull/4429).

Paired with https://github.com/matrix-org/complement/pull/849 and
https://github.com/matrix-org/sytest/tree/anoa/msc4429

Tracking issue for removing unstable identifiers in Synapse:
https://github.com/element-hq/synapse/issues/19891
Further improvements tracked in:
https://github.com/element-hq/synapse/issues/19981

---------

Co-authored-by: Half-Shot <will@half-shot.uk>
Co-authored-by: Jason Robinson <jasonr@element.io>
Co-authored-by: Olivier 'reivilibre' <oliverw@element.io>

docker/complement/conf/workers-shared-extra.yaml.j2

@@ -15,6 +15,8 @@ enable_registration_without_verification: true
 bcrypt_rounds: 4
 url_preview_enabled: true
 url_preview_ip_range_blacklist: []
+# MSC4429 Profile updates down legacy /sync
+include_profile_updates_in_sync: true
 
 ## Registration ##