JSON schema export

[peschmae]

As described in #898 I'll make a try to integrate JSON schema export into YTT.

It's based on my previous PR #899 since it's touching the same code, and I'd like to have the validation in the json schema.

I can detangle those PRs if required, but I think #899 will merged first, and I plan to rebase and squash anyways, before converting this to a proper PR.

Right now I'm mostly looking for feedback about the structure of the code, and some hints how I could improve it.
Since openApi schema is a superset of json schema, I already tried composition of the structs, but that doesn't work with the recursive nature of calculateProperties.
If there is some composition done, I would also argue, that the OpenAPIDocument should be based on JSONSchemaDocument (and the openAPIKeys type renamed to jsonSchemaKeys or similar), since that would reflect the set<->superset nature of json schema and openApiSchema .

[peschmae]

Changes in here all relate to #899

[peschmae]

Changes in here all relate to #899

[cppforlife]

probably worth rebasing on top of #904 (already merged).

quick & dirty json schema, needs a lot of refactoring, to make code DRY again

even though openapi and jsonschema is similar (and more recently openapi started using json schema in some parts) i actually dont have a problem with them being two separate code paths.

[peschmae]

Rebased onto develop, squashed the commits, and updated the tests, to use the same ytt schemas as the openapi tests.

Having played around with extracting the components from the openApi spec, into a json schema, I'm no longer sure, if it makes sense to have this as it's own export output type, even though I still like the idea to have validation in common text editors through json schema.

Maybe it makes more sense to keep the code in YTT simple, and just refere to another tool/script that can extract the components from the openApi schema into a json-schema, similat to what kubeconform does to get a json schema from a kubernetes CRD using https://github.com/yannh/kubeconform/blob/master/scripts/openapi2jsonschema.py

Stitching together two tools (ytt | openapi2jsonschema ) to get a json schema doesn't sound too bad.