Optimize and refactor ConfigService and GeneratedConfig

What does this MR do?

Various cleanup, refactoring, and performance optimizations in ConfigService and GeneratedConfig.

Tasks

• Remove unnecessary usage of constants (this is a documented anti-pattern, see https://docs.gitlab.com/ee/development/performance.html#moving-allocations-to-constants)
• Ensure GRPC calls are optimized and not duplicated. NOTE: There weren't any changes needed for this, the three existing GRPC calls (commit, markdown file blob, config file blob data) are all independent and necessary.
• Remove unnecessary nil checks of required values (for example, ref)

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Related Issues/MRs

• Depends on (and rebased on, until merged) !43384 (merged)
• Related to #251014 (closed)