Add CLI API for yaml validation / dry run

[matthewfeickert]

Following @ponyisi's talk on the ServiceX client v3.0.0 release in the 2024-09-24 IRIS-HEP Analysis Systems meeting it would be nice from a user perspective if there was a CLI API for doing schema validation of a ServiceX config.yaml file (reference: slide 7 from talk) given that the ServiceX client uses Pydantic and so things should be easy to validate.

An API that makes sense to me (without much thought on it though) would be something along the lines of

servicex validate [--config-file=<path>]

@jpivarski also pointed out that a "dry run" option would be useful. A dry run could validate both the syntax and the semantics and might even check to see if files exist, but not start any transformation or download any data.