Expose `select_exprs` on DataFrame

[timsaucer]

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

This is a user request to expose https://docs.rs/datafusion/latest/datafusion/dataframe/struct.DataFrame.html#method.select_exprs

Describe the solution you'd like

• Add method to PyDataFrame
• Add wrapper method
• Add unit tests

[milenkovicm]

#1271