function assignSyntheticsHeader(header, encKey, trustedIds, transaction) {
  const synthData = parseSyntheticsHeader(header, encKey, trustedIds)
  if (!synthData) {
    return
  }

  transaction.syntheticsData = synthData
  transaction.syntheticsHeader = header
}
Take the X-NewRelic-Synthetics header and apply any appropriate data to the
transaction for later use. This is the gate keeper for attributes being
added onto the transaction object for synthetics.

@param {string} header - The raw X-NewRelic-Synthetics header
@param {string} encKey - Encoding key handed down from the server
@param {Array.<number>} trustedIds - Array of accounts to trust the header from.
@param {object} transaction - Where the synthetics data is attached to.