Set priority on pipeline stage

### What feature do you want to see added?

Would it be possible to add support for setting priority on a specific stage in a pipeline? The use-case is having a pipeline that first executes a number of stages in parallel and then a final step once all the previous stages are done. The final stage doesn't take long, but it is queued at the end of the queue which may have a bunch of longer jobs already queue. Would be nice to be able to set a higher priority on the final stage to have the job complete faster.

### Upstream changes

_No response_

### Are you interested in contributing this feature?

_No response_

No files or tests are named in the issue, so start by tracing how this Jenkins plugin determines queue priority for pipeline work. Define how a priority on one stage should interact with parallel stages and the final stage, then verify that the final stage can be scheduled ahead of unrelated queued jobs without changing other pipeline behavior.

Written by the indexing model from the issue text.