Add a Redis store

[ghalse]

I've often wondered why the consent module doesn't have a Redis-based store. It's an obvious choice if you already use Redis-based sessions and have set it up for high availability.

I was writing some other Redis code and needed a challenge, so I thought I'd give it a go.

Full disclosure: what's here was produced with assistance from GitHub Copilot for the heavy lifting. But AI-generated code has all been manually reviewed (and changed!).

I've tried to keep it consistent with the way consent:Database works, but added inheritance of config from the global store.redis.* options in config.php. That makes more sense here because of how Redis itself works -- IMHO you're unlikely to have a second instance dedicated to this (although you might use a separate database slot).

[tvdijen]

Tagged v1.6.0