[C++][Dataset] Refactor Expression::Assume to return a Result

[asfimport]

Expression::Assume can abort if the two expressions are not valid against a single schema. This is not ideal since a schema is not always easily available. The method should be able to fail gracefully in the case of a best-effort simplification where validation against a schema is not desired.

#8037 (comment)

Reporter: Ben Kietzman / @bkietz
Assignee: Ben Kietzman / @bkietz

Related issues:

• [C++][Dataset] Minimize Expression to a wrapper around compute::Function (is superceded by)

_{Note: This issue was originally created as ARROW-9945. Please see the migration documentation for further details.}