Add support for redshift-serverless target

[esanchezm]

This could be a duplicate of #55 , which was closed to inactivity.

Is your request related to a problem? Please describe.

We want to use DMS to sync from a db to redshift-serverless. The module already offers a good solution for redshift targets, but the trust policy for the role must be different for this case

{
    "Sid": "",
    "Effect": "Allow",
    "Principal": {
        "Service": "redshift-serverless.amazonaws.com"
    },
    "Action": "sts:AssumeRole"
}

Describe the solution you'd like.

Probably adding a new variable enable_redshift_serverless_target_permissions like suggested in #55 that will determine the use of a different assume_role_policy in the resource "aws_iam_role" "dms_access_for_endpoint"

Describe alternatives you've considered.

Creating the dms-access-for-endpoint IAM role from outside but it's complicating things as I'm basically copy-pasting what's in the module 🗑️