Update secrets getter/setter methods and apply feature flags

What does this MR do and why?

Context

Since some job metadata will be moved to new destinations (e.g. intrinsic data moved to ci_builds and immutable processing data to ci_job_definitions) we need to be able to use feature flags to read/write data to specific destinations.

secrets is processing data that will be moved to ci_job_definitions.config.

This MR

Updates the secrets getter/setter, moves them to Ci::Metadatable and applies the following feature flags:

• read_from_new_ci_destinations (Roll out: #552063 (closed))
• stop_writing_builds_metadata (Roll out: #552065)

Also:

• Updates the EE build factory to set the value of secrets in both metadata and job_definition.
• Updates the affected RSpec tests to use :ee_ci_build and stubbed stop_writing_builds_metadata where neccessary.

FYI, for reference: The approach taken in this MR is very similar to the one for id_tokens in !202681 (merged).

References

• Resolves part of #552057 (closed)

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Related to #552057 (closed)