Add helper method to traverse node executions

[troychiu]

Why are the changes needed?

Currently it's hard for users to traverse all node executions under a workflowExecution/nodeExecution.

What changes were proposed in this pull request?

Added helper method for workflowExecution and nodeExecution.

How was this patch tested?

You can easily traverse nodes like

r = FlyteRemote()
execs = r.recent_executions(project=project, domain=domain, limit=limit)
a = r.sync(execs[0], sync_nodes=True)
for node in a.traverse_node_executions():
    print(node.id.node_id)

Example output:

n0
n0-0-n0
n0-0-n1
n0-0-n2
n0-0-n3
n0-0-n3-0-dn0
n1
n2
n3

Check all the applicable boxes

• [ ] I updated the documentation accordingly.
• [ ] All new and existing tests passed.
• [ ] All commits are signed-off.

Related PRs

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