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ARROW-9783: [Rust] [DataFusion] Remove aggregate expression data type

This is a step towards cleaning up the handling of aggregate expressions and achieves the following:

- It is now possible to construct a logical aggregate expression without specifying a return data type, so we can now just say `min(col("foo"))` for example.
- Removes the aggregate function methods from the `DataFrame` trait since they are no longer needed
- Removes duplicate logic from `expr_to_field` that was determining the data type of expressions rather than just calling `expr.get_type`
- Removes hard-coded return types from tests
- Adds the new `min`, `max`, `avg`, `count`, `sum` functions to prelude.rs

There is more to do in the future, such as allowing custom aggregate UDFs to be defined.

Closes #7988 from andygrove/remove-aggr-datatype

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