Redis Signal with Subscriptions.

[stmudie]

A Redis Signal based on #850 and the redis signal in production on the SAXS beamline at the Australian Synchrotron. It supports redis subscriptions if the redis server has notify-keyspace-events set to $K or AKE. There is also a Factory Class for returning keys for a specific Redis instance.

My first PR to Ophyd, definitely keen to get some feedback @tacaswell, @danielballan.

[danielballan]

Thanks for pushing on this, @stmudie! Will take look in detail later this week.

[mrakitin]

Power-cycling to restart the CI with GHA.

[mrakitin]

@stmudie, @tacaswell, can we consider this PR overriding https://github.com/bluesky/ophyd/pull/850?

[mrakitin]

Do you mind rebasing, or let me rebase it on master so that it could pick up the GHA-based CI testing? Thanks!