Add support for OAS 3.1.x in validator

[derevnjuk]

Description
Add support for OAS 3.1, this would allow validating API specifications that use the new features and improvements, such as callbacks and improved content negotiation.

Possible solution
The @har-sdk/validator can be updated to recognize the new syntax and semantics introduced in OAS 3.1 and modify the validation rules accordingly. However, it is not possible to use draft-2020-12 and previous JSON Schema versions in the same Ajv instance, so we should find out how to split and use them depending on the version. For details please refer to https://ajv.js.org/json-schema.html#draft-2020-12