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ARROW-13785: [C++] Add methods to print exec nodes/plans

Example output:

```
ExecPlan with 6 nodes:
SourceNode{"source", outputs=["filter"]}
FilterNode{"filter", inputs=[target: "source"], outputs=["project"], filter=(i32 >= 0)}
ProjectNode{"project", inputs=[target: "filter"], outputs=["aggregate"], projection=[bool, multiply(i32, 2)]}
GroupByNode{"aggregate", inputs=[groupby: "project"], outputs=["filter"], keys=["bool"], aggregates=[
	hash_sum(multiply(i32, 2)),
	hash_count(multiply(i32, 2), {mode=NON_NULL}),
]}
FilterNode{"filter", inputs=[target: "aggregate"], outputs=["order_by_sink"], filter=(sum(multiply(i32, 2)) > 10)}
OrderBySinkNode{"order_by_sink", inputs=[collected: "filter"], by={sort_keys=[sum(multiply(i32, 2)) ASC]}}
```

This prints nodes in the order of the topological sort. Since this is a graph and not necessarily a tree, I didn't try to indent the nodes themselves or anything.

Closes #11068 from lidavidm/arrow-13785

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>