This adds the new `required_plugins` block to be nested under the packer block.

Example:
```hcl
packer {
  required_plugins {
    aws = {
      version = ">= 2.7.0"
      source = "azr/aws"
    }
    azure = ">= 2.7.0"
  }
}
```

For example on darwin_amd64 Packer will install those under :
* "${PACKER_HOME_DIR}/plugin/github.com/azr/amazon/packer-plugin-amazon_2.7.0_x5.0_darwin_amd64"
* "${PACKER_HOME_DIR}/plugin/github.com/hashicorp/azure/packer-plugin-azure_2.7.0_x5.0_darwin_amd64_x5"

+ docs
+ tests