roachprod gc: enable cluster cleanup for multiple AWS accounts

[jlinder]

Problem:

roachprod clusters are now (or will be shortly) running in multiple AWS accounts. roachprod gc can only be configured to clean up one AWS account at a time. If roachprod gc is not extended to support cleanup of multiple AWS accounts on each run, then clusters that we want to be cleaned up will continue running and waste money.

Suggested solution:

1. Change roachprod gc to support cleaning multiple AWS accounts. Ideally, this would be done by providing an API access key pair to a single IAM service account that can assume role into the accounts with clusters to GC and a list of the accounts (as aliases) to assume role into. (A less desirable approach is to provide a list of access key pairs to the command, one per account to be cleaned.)
2. Change the roachprod-gc scheduled job to use the new feature to clean all the AWS accounts where we create and run roachprod clusters.

Jira issue: CRDB-31591

Epic DEVINF-235

[blathers-crl]

cc @cockroachdb/test-eng