[Python] Improve PyArrow support for Substrait ExtendedExpressions

[amol-]

Describe the enhancement requested

Substrait ExtendedExpressions can be used with PyArrow by converting them to compute.Expression.
That way they can be used in the context of projections and filters when interacting with a Dataset or Table.

Some pieces are still missing for those to be used in practice and the user experience is generally too complex to be convenient. This issue is meant to track work that can be done to improve the Substrait experience in PyArrow

• Allow accepting Substrait Message objects directly instead of bytes(currently pc.Expression.from_substrait(projection.SerializeToString()) dance is required which is not very convenient)
• Allow accepting substrait messages directly where a pc.Expression is accepted instead of having to build the expression from the message
• Have a way to encode PyArrow schemas to Substrait NamedStruct
• Accept projections as a single substrait ExtendedExpression instead of having to build multiple different expressions for each projected column.

Component(s)

Python