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.

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

See merge request !177