OpenAPI 3.0 support

[kibertoad]

An approach to investigating missing swagger features is a good question.
Suggested reading:

1. Supported data types
2. Validation for parameters
3. Security Defenitions

Those are probably the core features that related to input validation.

The approach in my opinion should be to start writing tests for each of the keywords and see the behavior, In case it is not as expected we can open an issue and fix it.

We also know that we miss support in multiple swagger file but that's a known issue that we shouldn't address now.

[idanto]

So now that we have support in openAPI 3.0, would you like to do some of those tests? @kibertoad

[kibertoad]

Sure!

[idanto]

I think we can move this list to api-schema-builder cause we get the support from the extracted package, or at least duplicate it there cause we probably would like to have few tests here.

wdyt @kibertoad ?

[kibertoad]

@idanto Makes sense!

[kibertoad]

Moved to PayU/api-schema-builder#23