executors: Security vetting

[efritz]

Vet the AWS and GCP Terraform modules to ensure that we are not introducing any gaping infrastructure holes, especially concentrating on networking and access from the public internet and IAM/roles.

The terraform module declarations to vet can be found at:

https://github.com/sourcegraph/terraform-aws-executors
https://github.com/sourcegraph/terraform-google-executors

These terraform modules will be (and are currently) used by us to deploy Cloud/Dogfood executors to both GCP and AWS, and these modules will also be made available to users to deploy them alongside their private instances.

Excuse the sparse READMEs at the moment - we're working on filling these out. Feel free to tag @efritz if you need more context or want a synchronous walkthrough/pair session to fully understand the usage here.

[github-actions]

Heads up @macraig - the "team/code-intelligence" label was applied to this issue.