Set TTS API keys

curl --request PATCH \
  --url https://api.ultravox.ai/api/accounts/me/tts_api_keys \
  --header 'Content-Type: application/json' \
  --header 'X-API-Key: <api-key>' \
  --data '{
  "elevenLabs": "<string>",
  "cartesia": "<string>",
  "playHt": "<string>",
  "lmnt": "<string>"
}'

{
  "elevenLabs": {
    "prefix": "<string>"
  },
  "cartesia": {
    "prefix": "<string>"
  },
  "playHt": {
    "prefix": "<string>"
  },
  "lmnt": {
    "prefix": "<string>"
  }
}