OpenAPI 3.1.0 support: add support for specification extensions in JSON Schema renderer

[char0n]

The new renderer supports registering renderers for custom JSON Schema keywords, as JSON Schema Draft 2020-12 can now be extended by custom vocabularies.

The OpenAPI 3.1.0 tells us the following about the Schema Object extensions:

This object MAY be extended with Specification Extensions, though as noted, additional properties MAY omit the x- prefix within this object.

So this basically means, we need to handle everything outside of default 2020-12 vocabulary as extension. Separate overridable
component needs to be introduced as part of oas31 plugin that will act like a fallback to all unrecognized schema keywords.