Adding keepdefaultrules option by means of a directory passthrough

What does this MR do?

Incorporporate a new option to the ruleset configuration that enables us to keep the default configuration as described in Discuss configuration options that enable users... (gitlab-org/gitlab#562142 - closed) • Julian Thome • 18.4 • On track

• 1️⃣ ruleset module (this MR) Adding keepdefaultrules option by means of a di... (!59 - merged) • Julian Thome • 18.5 (Adding support for keepdefaultrules)
  • 2️⃣ Semgrep MR Ruleset customization with default configuratio... (semgrep!630 - merged) • Julian Thome • 18.5
• 3️⃣ ruleset module Execute passthroughs selectively through Proces... (!62 - merged) • Julian Thome • 18.5 (Adding support for selective passthrough application)
  • 4️⃣ GLAS MR https://gitlab.com/gitlab-org/security-products/analyzers/gitlab-advanced-sast/-/merge_requests/218+s

What are the relevant issue numbers?

• Implement directory passthroughs in ruleset pac... (gitlab-org/gitlab#575923 - closed) • Julian Thome • 18.5 (main issue)
• Allow SAST custom rules to be appended to rathe... (gitlab-org/gitlab#426406 - closed) • Julian Thome • 18.5 • On track (secondary issue)
• Semgrep Integration: Ruleset customization with default configuratio... (semgrep!630 - merged) • Julian Thome • 18.5
• Test Project: https://gitlab.com/julianthome/keepdefaultconfig

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 updated/added for this feature/bug
• Job definition updated, if necessary
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  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer