Issue #, if available:

Description of changes:
This function can be used with AWS Directory Services or Windows Active Directory for LDAP related operations.
LDAP authentication done using a keytab file and a username.
This lambda must be placed in the same VPC as AWS Directory Services.
Also, DNS name must be enabled in the DHCP option set of the VPC so that the directory can be accessed using directory name. Also, add S3 endpoint as per https://aws.amazon.com/blogs/aws/new-vpc-endpoint-for-amazon-s3/

 Example JSON input :
         {
           "directoryName": "XXX.YYY.com",
           "userName": "USER",
           "baseDN": "DC=XXX,DC=YYY,DC=com",
           "keytabFileS3Url": "s3://KEYTABLOCATION/my.keytab",
            "filter": "(objectClass=*)"
        }

This is potentially a fix for #341

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