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ARROW-10547: [Rust][DataFusion] Do not lose Filters with UserDefined plan nodes

I have a LogicalPlan like this (where "Extension" is some extension node type):

```
    Extension [non_null_column:Utf8]
      Filter: #state Eq Utf8("MA") [city:Utf8;N, state:Utf8;N, borough:Utf8;N]
        InMemoryScan: projection=None [city:Utf8;N, state:Utf8;N, borough:Utf8;N]
```

When I run this plan through {{ExecutionContext::optimize}} the plan that results is as follows (note the filter has been lost):

```
    Extension [non_null_column:Utf8]
      InMemoryScan: projection=Some([0, 1, 2]) [city:Utf8;N, state:Utf8;N, borough:Utf8;N]
````

I have debugged the problem and the root cause of the issue is that the `FilterPushDown` logic is not recursing into the inputs of user defined nodes.

This PR has a fix for the problem

Closes #8633 from alamb/alamb/ARROW-10547-user-defined-plans

Authored-by: alamb <andrew@nerdnetworks.org>
Signed-off-by: Jorge C. Leitao <jorgecarleitao@gmail.com>