[Rust] [DataFusion] Add ability to downcast ExecutionPlan to specific operator

[asfimport]

Sometimes it is necessary to have operator-specific logic in a query optimizer, so we need the ability to get an Any reference to an ExecutionPlan so we can downcast it.

We do something very similar in PrimitiveArray already with an as_any method but the difference there is we can call get_type first to know exactly what type to cast to and we don't have anything like that in ExecutionPlan, but we could at least speculatively  try casting to specific operators.

Reporter: Andy Grove / @andygrove
Assignee: Andy Grove / @andygrove

PRs and other links:

• GitHub Pull Request #8284

_{Note: This issue was originally created as ARROW-9839. Please see the migration documentation for further details.}

[asfimport]

Andy Grove / @andygrove:
Issue resolved by pull request 8284
#8284