New attack technique: Usage of SSM StartSession on multiple instances

[adanalvarez]

What does this PR do?

• New attack technique

Just like https://github.com/DataDog/stratus-red-team/pull/467 the attack technique creates the same EC2 instances and its VPC in the warmup. In the attack phase, it uses StartSession to gain interactive access to multiple EC2 instances.

Motivation

Open Issue https://github.com/DataDog/stratus-red-team/issues/60

Checklist

• [x] The attack technique emulates a single attack step, not a full attack chain
• [x] We have factual evidence & references that the attack technique was used by real malware, pentesters, or attackers
• [x] The attack technique makes no assumption about the state of the environment prior to warming it up