Tracing: expose configuration to rabbitmq.conf

[michaelklishin]

This is only really relevant if the plugin stays in the distribution (#11553).

rabbitmq.conf schema supports path validators. The tracing plugin only has one configurable setting which is a path, and it can be validated to be writeable similarly to how TLS key and certificate paths are validated to be readable.

In fact, there's already a schema validator for that, dir_writable, which is used by the log file path setting.

See #11546 and #11551 for background.