Introduce Ci::Metadatable#timeout_value (renamed from metadata_timeout)

Refactoring

Use the changes from the Draft: PoC: split build metadata into `job_prot... (!193948 - closed) to rename metadata_timeout into Ci::Metadatable#timeout_value

Why?

This refactoring removes the explicit dependency on ci_builds_metadata table and instead moves the dependency to the abstraction Ci::Metadatable where we will have switch logic to read/write data to/from other sources like ci_job_prototypes and ci_builds.