Support enumerated values in SDK JSON schema helpers

Summary

Support enum in JSON Schema helpers.

Proposed benefits

Validation of enumerated values in records and package configurations.

Proposal details

Add an enum keyword argument to the Property constructor.

Docs: https://json-schema.org/understanding-json-schema/reference/generic.html#enumerated-values

Best reasons not to build

CustomType is a way to do this if users don't mind typing a raw JSON Schema dictionary.