Support sending and receiving MSC4354 Sticky Event metadata. (#19365)

Part of: MSC4354 whose experimental feature tracking issue is
https://github.com/element-hq/synapse/issues/19409

Follows: #19340 (a necessary bugfix for `/event/` to set this metadata)

Partially supersedes: #18968

This PR implements the first batch of work to support MSC4354 Sticky
Events.

Sticky events are events that have been configured with a finite
'stickiness' duration,
capped to 1 hour per current MSC draft.

Whilst an event is sticky, we provide stronger delivery guarantees for
the event, both to
our clients and to remote homeservers, essentially making it reliable
delivery as long as we
have a functional connection to the client/server and until the
stickiness expires.

This PR merely supports creating sticky events and receiving the sticky
TTL metadata in clients.
It is not suitable for trialling sticky events since none of the other
semantics are implemented.

Contains a temporary SQLite workaround due to a bug in our supported
version enforcement: https://github.com/element-hq/synapse/issues/19452

---------

Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org>
Co-authored-by: Eric Eastwood <erice@element.io>

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

@@ -139,6 +139,8 @@ experimental_features:
   msc4155_enabled: true
   # Thread Subscriptions
   msc4306_enabled: true
+  # Sticky Events
+  msc4354_enabled: true
 
 server_notices:
   system_mxid_localpart: _server