Backport feature flags subsystem to RabbitMQ 3.7.x

[dumbbell]

The goals behind this backport are:

1. Allow plugin developers to use the subsystem API in their plugins and still have plugins compatible with both RabbitMQ 3.8.x and 3.7.x (for 3.7.x releases including the subsystem).
2. Allow a RabbitMQ 3.7.x node to join a RabbitMQ 3.8.x cluster if this cluster has no feature flags enabled; this could be useful during an upgrade is a 3.7.x node has to be restarted for instance.

This backport includes the feature flags subsystem only! There are no feature flags as of this writing. We might decide to introduce feature flags in future 3.7.x release if there is a need, but this is a separate question.

This issue is created to track all the pull requests involved in the backport.

[#164026796]