roachprod: add support for AWS auto scaling groups

[srosenberg]

In [1], we added GCE's analogue of auto scaling groups, i.e., managed instance groups (MIGs). This has been instrumental during scale testing when the cluster had to be scaled up without tearing it down. In the future, we plan to run some scale tests in AWS. Thus, we should be able to provision auto scaling groups by then.

[1] #118423

Jira issue: CRDB-54175

[blathers-crl]

cc @cockroachdb/test-eng