curl --request POST \
  --url https://api.mpcvault.com/v1/executeSigningRequests \
  --header 'Content-Type: application/json' \
  --header 'x-mtoken: <api-key>' \
  --data '
{
  "uuid": "<string>"
}
'

{
  "error": {
    "executeSigningRequestsErrorCode": "EXECUTE_SIGNING_REQUESTS_ERROR_CODE_UNSPECIFIED",
    "message": "<string>"
  },
  "txHash": "<string>",
  "signatures": {
    "signatures": [
      {
        "ecdsaSignature": {
          "R": "<string>",
          "S": "<string>",
          "V": "<string>"
        }
      }
    ]
  },
  "signedTransaction": "<string>"
}