Add wrapper classes for Expression types #1681

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

As part of the discussion in #1674 we found a user wants access to some of the elements within an expression. In this case, Limit. If there is a growing base of users who want to use datafusion-python to do things such as analyzing the plans and using the data in them, then we should probably add wrapper classes for each of the Expressions.

Describe the solution you'd like

Right now we have the expressions like Limit available in datafusion.expr reexported like

Limit = expr_internal.Limit

Instead we could have a datafusion.expr module that contains

class Limit(Expr):

which has attributes to get fetch and skip.

Describe alternatives you've considered

We could leave as is and continue to expose data members as requested.

Additional context

This is a follow on to #1673