feat: add configuration to store enums as pg custom domain types

[prevostc]

Motivation: more efficient domain storage of enum data.

JSON Schema References:

• https://json-schema.org/understanding-json-schema/reference/enum
• https://www.rfc-editor.org/rfc/rfc8927#section-2.2.4
• https://www.rfc-editor.org/rfc/rfc8927#semantics-form-enum
• https://cswr.github.io/JsonSchema/spec/generic_keywords/#enumerated-values

Enums are stored as 1 or 2 bytes they are very space efficient when it's possible to use them.
Because enums have a lots of downsides too, this feature is behind a feature flag.

This can never be a full, specification accurate implementation, as a subset of valid jsonschema enum cannot map to a valid pg domain. Ex: {"enum": ["a", 3, null]} is a valid jsonschema.