Added an optional 'interface' property for Operation.

When available, this property should supersede the generated name for the Scala operation.

Example:

    "resources": [
      {
        "model": "organization",
        "operations": [
          {
            "method": "POST",
            "interface": "createOrganization",
            "description": "Create a new organization.",
            "parameters": [ ...

Instead of `postOrganizations`, this operation would be called `createOrganization` in the generated Scala client.