Provide access to source variables for all plugins.

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#### Description

These docs suggest that you can access variables defined on sources later in build blocks.
https://developer.hashicorp.com/packer/docs/templates/hcl_templates/blocks/source

But from all the examples in this discussion thread it appears you cannot do this with the amazon-ebs plugin.
https://discuss.hashicorp.com/t/packer-source-blocks-unable-to-validate-in-packer-1-11-2/71853/9

We cannot for example reference ssh_username in an ansible plugin provisioner which means we repeat ourselves.

It is also not possible to define extra source variables unless the plugin uses them. This would be very helpful. Currently the [documentation](https://developer.hashicorp.com/packer/docs/templates/hcl_templates/blocks/source) leads us to believe that these things are currently possible, but they are not.

#### Use Case(s)

To be able to reuse code without repe

#### Potential configuration

```
```

#### Potential References

Start with the source-block documentation and the linked discussion about Packer 1.11.2 validation, then compare amazon-ebs source variables with their use in an Ansible provisioner. Done means source variables and additional source-defined variables are consistently accessible across plugins, or the documentation clearly states the supported limits.

Written by the indexing model from the issue text.