Create SIP Registration

curl --request POST \
  --url https://api.ultravox.ai/api/sip/registrations \
  --header 'Content-Type: application/json' \
  --header 'X-API-Key: <api-key>' \
  --data '{
  "username": "<string>",
  "password": "<string>",
  "proxy": "<string>"
}'

{
  "registrationId": "<string>",
  "created": "2023-11-07T05:31:56Z",
  "username": "<string>",
  "proxy": "<string>"
}