[Backend] Add variable precedence controls for pipeline execution policies

Why are we doing this work

Based on the findings of the spike #520088 (closed), implement the support for variables control for pipeline execution policies.

The MR with this feature is prepared !186120 (merged). After we receive review from Verify, we'll add tests and proceed with it.

Relevant links

• Spike findings: #520088 (comment 2428519304)
• PoC MR: !186120 (merged)

Non-functional requirements

• Documentation:
• Feature flag:
• Performance:
• Testing:

Verification steps