API: Voices APIs changed

In view of the new Simba Turbo model launch we have made improvements to the voices APIs. The changes are as follows:

when GETting the list of voices the model and languages properties of the Voice object were merged into a single models property:

"models": [
  {
    "name": "simba-base",
    "languages": [
      {
        "locale": "en-US",
        "preview_audio": "https://storage.googleapis.com/speechify-ai-api-prod-centralized-voice-list/variants%2Fai-api/previews/en-US-victoria-speechify.mp3"
      }
    ]
  },
  {
    "name": "simba-turbo",
    "languages": [
      {
        "locale": "en-US",
        "preview_audio": "https://storage.googleapis.com/speechify-ai-api-prod-centralized-voice-list/variants%2Fai-api/previews/en-US-victoria-speechify.mp3"
      }
    ]
  }
]

similarly, when POSTing to clone the voice, the response voice details object got the model and languages replaced with the identical unified models property.