add support for SAST_SCANNER_ALLOWED_CLI_OPTS

What does this MR do?

• Add support SAST_SCANNER_ALLOWED_CLI_OPTS CI variable
• Add --max-memory flag under the list of allowed CLI options

What are the relevant issue numbers?

• Introduce a generic SAST CI var to forward flag... (gitlab-org/gitlab#368565 - closed)
• Accept --max-memory flag in SAST_SCANNER_ALLOWE... (gitlab-org/gitlab#369388 - closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer