Include Expr to datafusion::prelude

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

The datafusion prelude includes many helpful functions for constructing Exprs such as lit, col, etc: https://github.com/apache/arrow-datafusion/blob/master/datafusion/core/src/prelude.rs

I often find myself needing to use Expr as well, which for some reason is not included in the prelude but would be more convenient if it were.

Describe the solution you'd like
add Expr to prelude so instead of

use datafusion::logical_plan::Expr;

People can just do

use datafusion::prelude::*;

or

use datafusion::prelude::Expr;

Describe alternatives you've considered
Not doing it