Ability to access plugin version in HCL

#### Description

I use [packer.version](https://www.packer.io/docs/templates/hcl_templates/contextual-variables#packer-version) to add a description to my build including build time, packer version etc. Now that packer plugins are going to be abstracted, the plugin version is also important.

What I'm not sure of is should it be a contextual var depending on builder (`plugin.version`), or explicit var (`vmware_plugin.version`)

I pin the exact versions of the plugins, so maybe I could access this var as a workaround for now, if thats possible?

#### Use Case(s)

Aiding build reproducibility.

#### Potential configuration

```
vmx_data = {
annotation = "${var.description}|0A|0ABuilt with Packer ${packer.version} and vmware plugin ${vmware_plugin.version} on ${timestamp()}"
}
```