Common: add enrichment for default value substitution

[chuwy]

Right now we ignore JSON Schema default property because there's no best place to substitute it (in Enrich or during Loading). We can make it a configurable enrichment to fill some values with their default substitutions only for pre-configured schemas.

See https://discourse.snowplowanalytics.com/t/self-describing-event-jsonschema-defaults-dont-populate/4663