EBS source instance termination wait can dominate cleanup

### Overview

I think some Amazon EBS workflows could benefit from an explicit option to return after EC2 accepts source instance termination.

The case I measured used Packer to run work on a source instance and collect outputs before cleanup. The build did not need Packer to wait for the instance to reach the final `terminated` state.

### Measurements

In a direct EC2 lifecycle comparison, `c5.large` reached `terminated` in about one minute after `TerminateInstances` was accepted. `c5.metal` remained `shutting-down` for more than 19 minutes before later reaching `terminated`.

In a minimal local Packer run on `c5.metal`, EC2 accepted source instance termination around one second after Packer requested it. Waiting for the final state was the long part.

### Expected behavior

My theory is that the current default wait should stay unchanged. A separate opt in setting would let callers choose the faster completion point when their workflow does not need Packer to wait for final source instance deletion.

### Scope

This is separate from `skip_create_ami`. The option should not require image creation to be disabled. It is about the cleanup completion point after EC2 accepts source instance termination.