Add support for SSHKeyGen step from SDK

[LIV2]

Description

The Packer plugin SDK includes a step that will generate a temporary SSH Key for packer to use if no SSH Key or Password has been configured for the communicator.
The SSH Key is then cleaned up by the builder using the StepCleanupTempKeys step here

This is already implemented in the Virtualbox builder

Use Case(s)

This would streamline the build+provision process, the public portion of the key can be passed into the boot_command allowing a kicktart/preseed script to install the public key in the VM without needing to run any external commands to generate the key etc

Potential References

1. feat: Add SSHKeyGen Step from SDK #96
2. Virtualbox Builder documentation