feat(mandatory-jobs): provide AST-related CI/CD variable defaults

Add defaults for the AST_ENABLE_MR_PIPELINES and GITLAB_ADVANCED_SAST_ENABLED CI variables for mandatory jobs.

The default for AST_ENABLE_MR_PIPELINES mirrors the implicit default used by GitLab if no value is specified. Some projects make use of MR pipelines and so the corresponding pipeline execution policy which injects mandatory jobs should allow this variable to be overridden.

The default for GITLAB_ADVANCED_SAST_ENABLED is to enable advanced SAST. In rare cases a project may want non-advanced SAST and so the pipeline execution policy which injects mandatory jobs may allow this variable to be overridden if we deem it useful.