Schema DDL: add support for 'date' format

[miike]

This adds support for suggesting a Redshift DATE type when specifying "format": "date" in a JSON schema.

It's not entirely clear as to what is the best way to support a date with no time component as date was present in JSON schema v3 but removed in later versions. In v4 the spec mentions that anything with "format": "date-time" should meet section 5.6 of RFC3339 but the ABNF for "date-time" suggests that both a full date and a full time are required
date-time = full-date "T" full-time this differs from the ISO8601 which allows for partial dates. Thoughts @chuwy @alexanderdean?

[snowplowcla]

@miike has signed the CLA.

[alexanderdean]

Thanks for this @miike - I was schema'ing Northwind the other day and was wishing for a DATE mapping (https://github.com/snowplow/iglu-central/pull/611/files).

It's not entirely clear as to what is the best way to support a date with no time component as date was present

My view is that we should pretend format: date is still available in v4, and we should expect YYYY-MM-DD. Thoughts @chuwy?

[chuwy]

I absolutely agree - we should support it.

Even further, according to v4 - it is up to to validator app to implement custom formats. Which means we're absolutely free to add custom validators with formats like date, file-path, json-path, iglu-uri etc. to our schema-validation code. Third-party validators that don't understand these formats according to specification must simply ignore them, but in Snowplow/Iglu stack this should drastically simplify things and add some safety.

See #170

[alexanderdean]

Great, scheduling

[oguzhanunlu]

Cherry-picked to #280 .