[core] process TRACE operation

[jmini]

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Description of the PR

Fixes #3647

Before this change the trace operation under all PathItem is just ignored by OpenAPI-
Generator.

With this change, the operation is considered. As written on the issue, the generator maintainers might need to do some changes to produce correct code for the trace operation.

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[jmini]

Tested on the Java client generator (without any further changes to the templates) with a spec like this:

openapi: 3.0.1
info:
  title: OpenAPI Test
  description: Test Spec
  version: 1.0.0
  license:
    name: Apache-2.0
    url: 'http://www.apache.org/licenses/LICENSE-2.0.html'
servers:
  - url: 'http://localhost:8090/'
paths:
  /lorem/traceEmpty:
    trace:
      tags:
        - lorem
      operationId: emptyTrace
      responses:
        '201':
          description: OK

All java-client libs are producing code that compiles:

• java-feign
• java-google-api-client
• java-jersey2
• java-okhttp-gson
• java-rest-assured
• java-resteasy
• java-resttemplate
• java-webclient

And my first tests against a Mock-Server are also positive.

[wing328]

LGTM

[jmini]

And my first tests against a Mock-Server are also positive.

I spoke too quickly, I have found an issue with java-jersey2 and java-resteasy:
see PR #3653