fix: enrich type definition of buildCompilerFromPool method

[gianmarco27]

Enriches type definition for buildCompilerFromPool method to better represent fastify/ajv-compiler features.

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

[mcollina]

lgtm

[Eomm]

The TS error is unrelated with this PR

@fastify/typescript do you have a quick fix for this?
I saw it is related to this:
typescript-eslint/typescript-eslint#3325

I would like to fix it before publishing

[gianmarco27]

I have been reading through a set of issues regarding this false positive
(e.g. typescript-eslint/typescript-eslint#10266) but yet i didn't find any option to tune it.

I think however using declare there might actually be the issue, telling the compiler that this function already exists in the code, while buildCompilerFromPool does not actually exist in the code outside of the return scope of the AjvCompiler function.

So my proposal here would be to convert the two signature definitions to just types.

Follows commit (We can drop it if you disagree on this solution)