Feature request: provide json-schema for CSRJSON and config files

It would be useful having a [json-schema](https://json-schema.org/learn/getting-started-step-by-step) for `CSRJSON` and `config.json`

`cfssl print-defaults` is a useful command but the defaults may not contain all possible keys, types or list of possible values.

Using json-schema could for example describe possible regex patterns for `expiry`, object keys for each element of the `names` array or values for the `usages` array (as enumeration).

Validation is then simple: `jsonschema -i input.json schema.json` (e.g. using [this python validator](https://github.com/Julian/jsonschema))