New variables - assign public IP and target AMI name

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### Description

Add two new variables, `associate_public_ip_address` and `target_ami_name_prefix`.

### Motivation

The Packer EBS Builder includes a variable to force the association of a [public IP address](https://developer.hashicorp.com/packer/plugins/builders/amazon/ebs#associate_public_ip_address) to the builder. This is needed when using a non-default VPC that does not automatically assign public IP addresses. Exposing that variable to users would be beneficial.

The `ami_name_prefix` variable is currently misleading. The description is "The prefix to use when creating the AMI name", which is accurate, but this same prefix is used to locate the source AMI when building. This means the variable cannot be changed to customize the target AMI name. We can expose a second variable that is specifically used for the target AMI name and update the `ami_name_prefix` variable to more accurately describe its use.

### References

* [https://developer.hashicorp.com/packer/plugins/builders/amazon/ebs#associate_public_ip_address](https://developer.hashicorp.com/packer/plugins/builders/amazon/ebs#associate_public_ip_address)