Add support for user-defined keywords

[constanzaderienzo]

Is your feature request related to a problem? Please describe.
Given our business logic, we are in need of adding custom-keywords.

For example:

in: path
name: offset
schema:
    type: number
    myUserKeyword: true # Custom keyword

We found that AJV has this feature: https://ajv.js.org/guide/user-keywords.html#user-defined-keywords.

However, there is no way to extend the Ajv instance used by the validator (or at least we couldn't find one).

Describe the solution you'd like
Support for custom-keywords.

A solution is provided in this PR

[cdimascio]

@constanzaderienzo can you describe the current behavior today and the desired behavior if this change were implemented and merged?