Support passing in a custom Ajv instance

[felixfbecker]

Prerequisites

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🚀 Feature Proposal

Please provide the ability to pass in your own Ajv instance.

Motivation

Currently, one can only pass in ajv options, and the plugin instantiates ajv.
But to use latest drafts of JSON Schema, one has to import a different Ajv class than the default export: https://ajv.js.org/json-schema.html#draft-2020-12

In Fastify request validation, this is possible with setCompileFactory(), but it won't work for response validation.

Example

const ajv = new Ajv2020({ ... })
fastify.register(responseValidation, { ajv })

[mcollina]

Thanks for reporting! Would you like to send a Pull Request to address this issue? Remember to add unit tests.

[Fdawgs]

Closed by #94