First of all: Happy holidays! :)

The reason for this PR stems from #6223

Below i enumerate the two issues i found, with a bullet list:

• Currently, in a TS project if I try to use the onCreate ajv option I still get met with a typescript error (requiring a ts-expect-error), as such option is not defined in the exposed fastify type, although made available by fastify/ajv-compiler
  (My suggested fix is to mirror the entire ajv options field type definition from the ajv-compiler itself, as fastify already proxies the object to it in the js implementation, this way the type definitions remain transparent to the implementation and future updates)

• Additionally I have added also a test to showcase an error in the current ajv plugin type definition where the override of the plugin types is too generous, and allows for plugins to be passed, that would break the ajv plugin definition fluent standard, so if i provide my own validatorCompiler, leveraging this Ajv feature, i get no warning from TS to alert me that perhaps these plugins might break my implementation because they are not returning the Ajv instance.
  (Again proxying the type directly from the library solves this as it enforces the Ajv plugin definition)

I have committed types tests showcasing the 2 different use cases above, that would fail with the current type definition and following them up with the related fix, together with the dependency update of fastify/ajv-compiler that is required to apply these improvements.

Lastly I have added a small enrichment in docs for the ajv options and a little fix in jsdocs for fastify which i found while browsing the js code.

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct