support for string duration format

[khelnagar]

simply generate fixed length string {10} that contains 0-9 digits https://json-schema.org/understanding-json-schema/reference/string.html#dates-and-times

[ghandic]

Single digit doesn't seem to cover all of ISO 8601

https://datatracker.ietf.org/doc/html/rfc3339#appendix-A

[ghandic]

"For example, P3D expresses a duration of 3 days."

[ghandic]

Moved the PR to this one to avoid all of the formatting issues https://github.com/ghandic/jsf/pull/49/files