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:

1 "models": [
2   {
3     "name": "simba-base",
4     "languages": [
5       {
6         "locale": "en-US",
7         "preview_audio": "https://storage.googleapis.com/speechify-ai-api-prod-centralized-voice-list/variants%2Fai-api/previews/en-US-victoria-speechify.mp3"
8       }
9     ]
10   },
11   {
12     "name": "simba-turbo",
13     "languages": [
14       {
15         "locale": "en-US",
16         "preview_audio": "https://storage.googleapis.com/speechify-ai-api-prod-centralized-voice-list/variants%2Fai-api/previews/en-US-victoria-speechify.mp3"
17       }
18     ]
19   }
20 ]

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