add unknown => empty mapping in JTDSchemaType

[erikbrinkman]

What issue does this pull request resolve?
Makes sure that unknown types can be validated with an empty schema

What changes did you make?
Added the above change, and also some tests, notably a test that shows that typescript doesn't error when specifying the discriminator again in the schema (although I'm not sure why). And a test of a recursive schema.

Is there anything that requires more attention while reviewing?
should be pretty straightforward

[epoberezkin]

I had to revert it - I added JTDSchemaType to compile/validate type signatures and some minimal test for it, but this PR fails type inference in another place - here - see this error. Need to look into it later or maybe you can...

Also, strangely "compile" works without explicit JSONSchemaType annotation (see here) but JTDSchemaType required explicit type annotation (see here) - any idea why?

[epoberezkin]

cc @erikbrinkman ^^