Support Rails 8+ Solid Cache

[fallwith]

With rails/rails#50443, Rails is exploring the use of Solid Cache as a default.

We should track progress to that end and make sure we provide great instrumentation for Solid Cache. It's possible that the combination of our existing Rails notifications subscriptions and ActiveRecord instrumentation already provide the type of data we need.

In any case, we should test the agent with Solid Cache to establish a permanent playground app with Solid Cache and determine what information we do and do not offer once it's in place.

[workato-integration]

https://new-relic.atlassian.net/browse/NR-294498