Implement mercure-bundle #625
Description
The bundle should install both the DI and mercure components and wire up services/aliases and such via a dedicated MercureBundle, if enabled.
One somewhat option question is what namespace this should live under? Athena? Top-level?
Might want to block this by #614 given this'll be making use of ENV vars to configure it quite heavily.
Is possible to implement this properly, we'll need some new DI bundle schema methods. Mainly if we want to support multiple named hubs. Similar as for configuring messages busses, I'm not sure we have a way to handle hash-like named collections.