Duration format

[WillAbides]

version: v0.20.0

String fields with format: duration are generated as time.Duration. time.Duration marshals to an integer (nanoseconds) instead of a string.

jsonschema 2020-12 defines the duration format as an ISO 8601 duration string.

I see this repo is already parsing duration examples correctly using github.com/sosodev/duration, but it still needs to generate json marshalers and unmarshalers that do the same.