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?

[vorobeybird]

Hi @cdimascio, we have the similar issue. We would like to keep the specification as single source of truth, to avoid drift between the specification and initial code implementation. And keywords look like the perfect solution for that.