feat: add associated realtime feeds to GTFS Schedule response(OpenApi)

[davidgamez]

Summary:

Part of #420 Added gtfs_rt_feeds to the GTFS OpenApi entity. This affects endpoints: gtfs_feeds, gtfs_feeds/{id} and search

Expected behavior: The gtfs_rtfeeds field is added to the OpenApi model. Implementation is out of the scope of this PR. Example of the "related gtfs rt feed"_

...
"gtfs_rt_feeds": [
      {
        "id": "mdb-1210",
        "status": "deprecated",
        "provider": "Los Angeles Department of Transportation (LADOT, DASH, Commuter Express)",
        "feed_name": "Bus",
        "source_info": {
          "producer_url": "https://ladotbus.com/gtfs",
          "authentication_type": 2,
          "authentication_info_url": "https://apidevelopers.ladottransit.com",
          "api_key_parameter_name": "Ocp-Apim-Subscription-Key",
          "license_url": "https://www.ladottransit.com/dla.html"
        },
        "entity_types": [
          "vp"
        ]
      }
...

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