Add support for OpenAPI 3.1 nullable patterns in time field validation

[jonchun]

Related to #21: The aep-142-time-field-type rule fails to validate nullable timestamp fields in OpenAPI 3.1 specs.

In OAS 3.1, nullable fields are represented using anyOf with a null type. e.g.

delete_time: {
  anyOf: [
    { type: string, format: date-time },
    { type: null }
  ]
}

The current validation checks field.type directly, which is undefined for anyOf structures. This causes false positives for valid nullable _time fields. (such as a delete_time field for soft-deletes)

[jonchun]

Re-opening #71 from a proper branch. @toumorokoshi @mkistler

[mkistler]

It's not clear to me that we want to allow nullable date/time fields in AEP compliant APIs. I don't see any mention of "null" in AEP 142 currently. If we intend "null" to be meaningful in date/time fields, I think the AEP should state that explicitly.

[jonchun]

That's a fair point. I do think that AEP-142 is a bit too strict then, but that's to be taken up with AEP.

Workaround is clearly documented for anyone who has the same issue I did:

overrides:
  - files:
      - 'openapi.json#/components/schemas/book/properties/expiration'
    rules:
      aep-142-time-field-suffix: 'off'