Support for `mac1.metal`/`mac2.metal` instances

[thheinen]

:frowning_person: Problem Statement

I want to test some Mac-specific implementations with AWS.

:grey_question: Possible Solution

Add the necessary logic to select mac-AMIs and wire it to the corresponding types.

As mac-instances need to be on a dedicated host and are billed 24 hours minimum, this needs some dedicated host logic (see my issue #590)

:arrow_heading_up: Describe alternatives you've considered

No alternatives currently, as AMI selection and other logic is missing.

:heavy_plus_sign: Additional context

I already have an almost-working implementation for this. Will do some cleanups and PR after #590

[mfortin]

What's the status ? Host tenancy is one of the possible configuration, however there are alternatives, like using using a placement host_group_arn and license_configuration, can this also be supported ?