Add immediate trigger of pipeline execution schedule policy

Why are we doing this work

From &14147 (comment 2453223786)

It could be useful to be able to trigger an immediate "test". Even for our own purposes, creating a schedule and having to wait an hour, or go granularly specify some time and try to time it right, is sort of difficult. Out of scope for the immediate plan but I could see this being really useful 🤔

Relevant links

Non-functional requirements

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

Implementation plan

Verification steps