Skip inferred properties

[czechboy0]

Motivation

Fixes #379.

Context:

If you define an object schema as follows:

Foo:
  type: object
  properties:
    a:
      type: string
  required:
    - b

OpenAPIKit will parse it as having two properties: a of type string and b being a fragment, with a optional and b required.

Unfortunately, often property names appear only in the required list as a result of a typo, leading to non-compiling code and a difficult debugging story.

It comes down to the fact that Swift OpenAPI Generator never officially supported defining properties this way, and continues not to.

If adopters want to do this intentionally, they should do this:

Foo:
  type: object
  properties:
    a:
      type: string
    b: {}
  required:
    - b

Modifications

This PR tightens the rules around inferred properties:

1. Inferred properties are skipped, so the result of the first example will only have the property a generated, and the second example both a and b.
2. The generator emits a warning diagnostic when encountering an inferred property, which should help adopters who made a typo catch it much earlier in the process.
3. Bumps OpenAPIKit to 3.1.0, as the inferred property was added just yesterday.

Result

Less likely typos and mismatch between properties and required, and a diagnostic to help debug this case.

Test Plan

Added a snippet test.

[czechboy0]

@swift-server-bot test this please