[Rust][DataFusion] Add support for AVG(Timestamp) types

This is a follow on to ARROW-12277

Background: Support for Min/Max/Sum/Count were added for DataType::Timestamp types in https://github.com/apache/arrow/pull/9970.

This ticket tracks adding support for Avg, which is slightly more involved as currently Avg assumes the output type is always F64, and in this case I think Avg(timestamp) should also be (timestamp). We should double check what postgres does in this case and follow its example