Add warning when detonating a slow attack technique

[christophetd]

Attack techniques marked as "slow" can be surprising for first-time users as they can take a long time to detonate.

Sample output:

$ stratus detonate aws.exfiltration.rds-share-snapshot
2022/07/14 09:26:06 Checking your authentication against AWS
2022/07/14 09:26:06 Note: This is a slow attack technique, it might take a long time to detonate
2022/07/14 09:26:06 Warming up aws.exfiltration.rds-share-snapshot
2022/07/14 09:26:06 Applying Terraform to spin up technique prerequisites