[swagger-parser-v2-converter] vendor extensions from body params not copied to RequestBody

Vendor extensions defined on body params are not copied over to the OpenAPI v3 RequestBody

Example:

{
            "in": "body",
            "name": "body",
            "required": true,
            "schema": {
              "$ref": "#/definitions/Person"
            },
            "x-log-mapping": {
              "$.name": "LAST_NAME",
              "$.firstName": "FIRST_NAME"
            }
}

Start in the swagger-parser-v2-converter body-parameter conversion path and trace how the example body parameter becomes an OpenAPI v3 RequestBody. Check how vendor extensions are handled during that conversion. Done means the x-log-mapping extension from the example is present on the resulting RequestBody, with a regression test covering it.