Add custom actions for validate use case

[benc-uk]

As per the groups design session
https://github.com/aztfmod/rovergo/wiki/Groups-Design-Session

The following external tools and custom actions need to be added for the "validate / ci" command chain

• Chekov - https://github.com/bridgecrewio/checkov
• Tfsec - https://github.com/tfsec/tfsec
• Detect-secrets - https://github.com/Yelp/detect-secrets
• Tflint - Done, perhaps include opinionated config for enabled rules
• Caflint - Will this be external or eventually be run as a special type of internal action? @hattan ?

Reasons

• Required to run CI in piplines
• To support the chained command group

Notes / Considerations

• Probably will need a lot of changes to the devcontainer dockerfile

Outcomes

• Tools added and installed into dev container
• Default custom actions for each added to pkg/custom/source