Enhance Builder Selection with Multi-Builder Filtering and Validation

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#### Description

Currently, when working with multi-cloud or multi-builder Packer templates, users rely mainly on the `-only` flag to select specific builders. There is no native mechanism to filter builders by tags, metadata, or capabilities.
This makes large or modular templates harder to manage, especially in CI/CD environments where dynamic builder selection is needed.

This feature request proposes **Enhanced Builder Selection with Multi-Builder Filtering and Validation**, enabling tagging, metadata-based filtering, and improved visibility of which builders will execute.

#### Use Case(s)

* Large multi-cloud templates containing AWS, Azure, GCP, VMware, and Docker builders.
* CI/CD pipelines needing conditional builder execution (e.g., build only `prod` images).
* Modular templates where builder selection depends on architecture, region, or environment.
* Avoiding accidental builds on unwanted builders.
* Improving template maintainability and clarity by grouping related builders.

#### Potential configuration

```hcl
builders = [
{
type = "amazon-ebs"
tags = ["prod", "x86", "us-east"]
},
{
type = "azure-arm"
tags = ["dev", "x86"]
},
{
type = "googlecompute"
tags = ["prod", "arm64"]
}
]

# CLI usage examples:
# Select builders with matching tags:
packer build -filter="tags=prod,x86"

# Filter by custom metadata:
packer build -filter="region=us-east"

The issue names the existing -only builder-selection flag but no files, tests, or entry points. Start by locating its implementation and related validation tests, then clarify the configuration syntax, filtering semantics, and validation behavior before defining what successful builder selection and visibility should look like.

Written by the indexing model from the issue text.