Move task event message logic into the API

[DingoEatingFuzz]

Currently the alloc-status command has a 100+ line decision tree for determining what message to show given the shape and state of a task event.

The UI duplicates this logic for the allocation detail page.

alloc-status source
Nomad UI source

This should be moved up into the API so the two implementations won't drift and so other API consumers can benefit from simple message property.

[github-actions]

I'm going to lock this issue because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.