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

[alamb]

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

[alamb]

I am not sure what was going on with this variable name, but I figured I would reduce the replication ;)

[alamb]

More generally, I am not sure how / if this filter pushdown logic will work if there are multiple child inputs as no existing LogicalPlan variant his multiple inputs at this time. We will have to handle this when / if we want to support joins as well . For now I returned a "Not Yet Implemented" type error

[jorgecarleitao]

I agree :)

[github-actions]

https://issues.apache.org/jira/browse/ARROW-10547

[jorgecarleitao]

LGTM. Nice catch.