First impl

[Eomm]

@mcollina would you mind to give it a check if you spot potential issues in code?

I implemented this logic: https://json-schema.org/draft/2019-09/json-schema-core.html#rfc.appendix.B.1
It replaces all $ref with a new path to local /definitions added by code

It supports draft-07 (supported by ajv) with a small tweak to add draft-08 in future.

I tried it roughly with fastify-swagger and it does the job

TODO

• docs
• more test?

[mcollina]

The algorithm seems ok, I would add some docs and then we should be good to go.

(there will be bugs, but it should unblock us to ship Fastify v3)