Add variable to SAST vendored template to enable customer opt-in

Release notes

Problem to solve

IaC scanning in SAST needs a mechanism by which customers can opt in to using it.

Proposal

Introduce a new variable in the SAST template that can be used for all IaC scanners.

Proposed implementation plan

• Introduce a new variable in the SAST vendored template: SAST_IAC.
• Extend kubesec detection rules to use the new variable.
• Deprecate SCAN_KUBERNETES_MANIFESTS template variable in the SAST vendored template.
• Update existing kubesec tests to utilize new SAST_IAC variable`
• Update SAST analyzer documentation.