Tracing: expose directory path setting as tracing.dir

[michaelklishin]

in rabbitmq.conf using the same validator log.dir uses, and a similar configuration key:

tracing.dir = /path/to/a/writeable/directory

The error produced when the directory specified is not writeable looks like this:

[error] <0.164.0> tracing.dir invalid, Directory must be writable
[error] <0.164.0> Error preparing configuration in phase validation:
[error] <0.164.0>   - tracing.dir invalid, Directory must be writable

Note that the path setting is not covered by the tests because the test would have to use a writeable directory, and it is not obvious what kind of cross-platform path that is not computed programmatically they could use.

It's a trivial schema file that uses an existing core validator => let's leave it as is.

Closes #11554.

[razvanphp]

Thank you very much for taking the time to do this, really appreciated.