The identifier of the event source to be added. An identifier must begin with a letter and must contain only ASCII letters, digits, and
            hyphens; it cannot end with a hyphen or contain two consecutive hyphens. Constraints: 
- If the source type is a DB instance, then a DBInstanceIdentifier must be supplied.
- If the source type is a DB security group, a DBSecurityGroupName must be supplied.
- If the source type is a DB parameter group, a DBParameterGroupName must be supplied.
- If the source type is a DB snapshot, a DBSnapshotIdentifier must be supplied.
 Declaration Syntax
 Declaration Syntax| C# | 
public string SourceIdentifier { get; set; }