Add workflow events for suspended nodes

# Summary

Currently there are `node completion` events https://github.com/argoproj/argo/blob/master/docs/workflow-events.md:
* `WorkflowNodeSucceeded`
* `WorkflowNodeFailed`
* `WorkflowNodeError`

This proposal is to add an additional one for `WorkflowNodeSuspended`

# Use Cases

This would be used for an application that is managing the suspended nodes and responsible for resuming them.

---

**Message from the maintainers**:

Impacted by this bug? Give it a 👍. We prioritise the issues with the most 👍.

Start by reading docs/workflow-events.md and tracing how the existing WorkflowNodeSucceeded, WorkflowNodeFailed, and WorkflowNodeError events are defined and emitted. Confirm the suspended-node lifecycle and its tests before making the change. Done means WorkflowNodeSuspended is emitted for suspended nodes and documented alongside the existing node completion events.

Written by the indexing model from the issue text.