Add datasource_template_file_list

[PaulAtkins]

Adding datasource_template_file_list. This is similar to datasource_template_file and resource_template_dir, except instead of creating a rendered file, it returns an overall hash of the entire directory, plus a list of:

• file hash (as id)
• file name
• file contents

This allows the file contents to be used in other resources.

Example use case is the AWS ssm_document resource. When dealing with many of these documents, instead of creating a ssm_document for every document, it's easier to pass in a list of documents.

Example usage:

data "template_file_list" "list" {
	source_dir = "./docs"
}
output "files" {
	value = "${data.template_file_list.list.files}"
}

Previously, I was using the powershell below to retrieve this list, but this seems a cleaner approach, as it's not reliant on an external executable:

data "external" "doc-list" {
  program = ["powershell", "New-Object PSObject | Add-Member -PassThru NoteProperty Name ((Get-ChildItem -Path ../../docs/linux -File -Filter '*.json').Name -join ',') | ConvertTo-Json"]
}

Not sure if this is the correct provider / naming for this, or if it should be under external