Add ability to get latest revision of a service

Contents (#587)

New features

• Add method for getting latest SRUID for a service
• Use latest service revision in Service.ask when revision tag not provided or latest is used
• Allow specification of service registries in service configuration and via Child and Service

Enhancements

• Add function for splitting SRUIDs

Refactoring

• Rename functions in service_id module to remove redundant "service" from name

Dependencies

• Swap deprecated pkg_resources for importlib.metadata

Testing

• Factor out mock service revision tag and use instead of latest in tests

Style

• Use latest commit message checker