Proposed Feature

Spawned from #4702

This proposal is to:

1. Introduce a status.metadata on a Stage to hold user-defined metadata
2. Add the ability to set or read this metadata on a stage in a promotion step with a set-stage-metadata step and stageMetadata() helper respectively.
3. Add the ability to read this metadata when invoking an AnalysisTemplate, and in promotion steps.

NOTE: we already have a freightMetadata() helper, so if we are following that precedent, we would add a stageMetadata() helper.

Motivation

Verification is a process that is considered independent of promotion, and we don't have a good way of passing along information between those two processes. By introducing status.metadata, the promotion could set metadata on a Stage itself, so that it can be read by another process (e.g. verification or even a subsequent promotion).