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ARROW-8197: [Rust] [DataFusion] Fix schema returned by physical plan

This PR fixes a bug in the physical plan where schemas always had columns named c0, c1, ... and now returns the actual name of the expression used to create the result column. Because this could be a breaking change, especially for aggregate queries where the default column names are currently MIN, MAX, etc, I also added support for aliased expressions in both the logical and physical query plans so that it is possible to alias aggregate columns to avoid duplicate column names in a schema.

I have filed https://issues.apache.org/jira/browse/ARROW-8204 to add support for aliased expressions in SQL.

I also filed https://issues.apache.org/jira/browse/ARROW-8205 to add validation to enforce that schemas have unique field names.

Closes #6703 from andygrove/ARROW-8197

Authored-by: Andy Grove <andygrove73@gmail.com>
Signed-off-by: Andy Grove <andygrove73@gmail.com>