[Python] Make dataset Expression objects serializable

[asfimport]

It would be good to be able to pickle pyarrow.dataset.Expression objects (eg for use in dask.distributed)

Reporter: Joris Van den Bossche / @jorisvandenbossche
Assignee: Krisztian Szucs / @kszucs

Related issues:

• [C++] Implement LogicalPlan and LogicalPlanBuilder (is blocked by)

PRs and other links:

• GitHub Pull Request #6702

_{Note: This issue was originally created as ARROW-8060. Please see the migration documentation for further details.}

[asfimport]

Ben Kietzman / @bkietz:
this should probably wait for ARROW-7878 (and follow-ups) which includes the replacement for this class

[asfimport]

Neal Richardson / @nealrichardson:
@fsaintjacques I think we determined that this should not be blocked by LogicalPlan, right?

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 6702
#6702