fix: Use openapi-types 3.1 document type to support null schema types

[nidhi-kala]

Fixes #705

Type '"null"' is not assignable to type 'NonArraySchemaObjectType | undefined'.

By using OpenAPIV3_1.Document instead of OpenAPIV3.Document as type for Fastify dynamic swagger options, openapi config.

Checklist

• run npm run test and npm run benchmark
• 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

[Uzlopak]

I guess it is OK. But see my other remark.

[climba03003]

Please use OpenAPIV3 | OpenAPIV3_1 syntax. OpenAPI 3.1 is not a superset of OpenAPI 3.0