Ajv serializer

[Eomm]

Opening this draft to get feedback.
Right now this feature works, it needs more testing (CI fails due to the coverage).

The notable code is on the index and serializer-compiler files, the validator is just a copy-paste

AJV-JTD can be used to serialize the JSON payloads: https://ajv.js.org/guide/getting-started.html#parsing-and-serializing-json

I'm not sure if it is good to include this feature in this package, or we should create a new package that:

• support fast-json-stringify by default
• let the user opt-in to ajv-JTD feature

What do you think?

Closes fastify/fastify#3441

[jsumners]

I don't see any issue with this.

[dalisoft]

@Eomm Any performance comparison would be helpful
At least here for users could see how performance differs than fast-json-stringify

[mcollina]

I would call the option jtdSerializer:

[Eomm]

Must add small benchmarks

[mcollina]

lgtm

[jsumners]

Looks good to me.