Exclude configuration definitions from json_schema serialization

Problem/Motivation

There is a special case in Drupal\json_schema\Normalizer\DataReferenceDefinitionNormalizer to exclude a couple configuration data definitions from processing. For purposes of the JSON Schema serializer, these are not helpful and a cleaner means to skip them than an ad hoc blacklist would be helpful to avoid other config elements from sliding in.

Proposed resolution

Identify a common interface or attribute of config data definitions that can be used as a more global check for configuration-ness.

Remaining tasks

All the things!

User interface changes

N/A

API changes

N/A

Data model changes

N/A