WIP: Use santhosh-tekuri/jsonschema

[lkm]

This is based of this PR #318 to remove internal validation and originally replace with xeipuuv/gojsonschema and then in the newest commits replace with santhosh-tekuri/jsonschema

I'm using an extension (openapi3/components_extension.go to validate all components.schemas in one go as this seems to be the only way to keep internal document referencing working, e.g. #/components.schemas.ExampleObject.

This is very much still a work in progress and I'd need to resolve the fact the for this to work fully within the openapi3filter we'll need the complete openapi3.T document to resolve request and response references too. However, that very much seems to go against the grain of the current design.

I could use some input or opinions on direction. @fenollp

[lkm]

Another idea would be to use the OAS json schema to validate the entire document, see https://github.com/OAI/OpenAPI-Specification/blob/main/schemas/v3.1/schema.json - however, that may be too much tie-in and too big of a diversion as we'd have to write a few extensions to handle the different aspects.