Dynamic data blocks

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#### Description

It would be nice to be able to define `dynamic` top-level `data` blocks.

#### Use Case(s)

Let's say I need to build similar images across many AWS regions. Specifically, this involves looking up base AMI per-region. At the moment this can be achieved by generating source blocks dynamically within a build block, something like:

```
variable "regions" {
default = ["us-east-2", "eu-central-1"]
}

source "amazon-ebs" "common-ami-props" {
}

build {
dynamic "source" {
for_each = var.regions
labels = ["amazon-ebs.common-ami-props"]
content {
source_ami_filter {
...
}
region = source.value
}
}
}
```

This works because we use `source_ami_filter` within our source blocks. Introduction of `data` blocks, however seems to discourage that. Quoting the [announcement block post](https://www.hashicorp.com/blog/announcing-the-packer-data-source-component-type):
> Both configuration parameters will remain available for use in Packer build configuration. However, we encourage you to update your configuration to the new data source configuration for future stability.

The problem is that it's not possible to build `data` blocks dynamically with `for_each`. Neither is it possible to inherit `region` in some other way. You'd have to duplicate definition on `data` block for each region, [much like is done in official examples](https://learn.hashicorp.com/tutorials/packer/golden-image-with-hcp-packer#review-golden-image-configuration).

P.S. This is fairly similar to #11301, except that it is for `data` blocks and not `build` blocks.

Start from the dynamic source-block example and the stated limitation that top-level data blocks cannot use for_each. Compare the requested behavior with the related build-block issue #11301 and the linked data-source announcement. Done means data blocks can be defined dynamically for multiple regions without duplicating each regional definition.

Written by the indexing model from the issue text.