add primitive factory method for string/date

[jeremyhayes]

Description

Adds an example value provider for "date" format strings.

Given:

"properties": {
   "myDateField": {
      "type":"string", 
      "format":"date"
   }
}

Should provide an example ISO-formatted date-only string, such as

"2018-11-13"

Motivation and Context

Previously, these fields would be represented in request/response "Example Value" payloads as simply "string". This change provides semantic context, similar to other primitive values (number, boolean, etc).

How Has This Been Tested?

Added unit tests to cover sample values for both "date" and "date-time" formats.

Screenshots (if appropriate):

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• My changes can and should be tested by unit and/or integration tests.
• If yes to above: I have added tests to cover my changes.
• If yes to above: I have taken care to cover edge cases in my tests.
• All new and existing tests passed.

[shockey]

💪

[shockey]

thanks @jeremyhayes!