Values.MasterUserOptionsSourceCredentials for the master user: username and password, ARN, or both.
type nonrec t = {masterUserARN : ARN.t option;ARN for the master user (if IAM is enabled).
*)masterUserName : Username.t option;The master user's username, which is stored in the Amazon Elasticsearch Service domain's internal database.
*)masterUserPassword : Password.t option;The master user's password, which is stored in the Amazon Elasticsearch Service domain's internal database.
*)}