roachprod: add load balancer support (AWS)

[srosenberg]

In [1] we added support to spin up a load balancer in GCE, when using MIGs. We should do the same for AWS. Note that this in support of future scale testing in AWS. The load balancer likely depends on the auto scaling groups [2].

[1] #119382
[2] #153071

Jira issue: CRDB-54176

[blathers-crl]

cc @cockroachdb/test-eng