Fetch and store CI secrets configuration when processing CI yaml

What does this MR do?

This MR is part of series of dependent MRs extracted from !33231 (closed) in order to split work in smaller chunks:

• Add column to ci_builds_metadata to store secrets !34480 (merged)
  • ci_secrets_management feature check !34587 (merged)
    • Add check for "secrets" runner feature !34813 (merged)
      • (this MR) Yaml processing and saving secrets configuration in DB !34896 (merged)
        • Expose secrets configuration in API response !34898 (merged)

It builds on top of !34813 (merged) and updates Gitlab::CI:YamlProcessor to fetch and store CI secrets configuration in ci_builds_metadata.secrets.

Related to #28321 (closed) and #218746 (closed).

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team