ARROW-8060: [Python] Make dataset Expression objects serializable

[kszucs]

The Scalar are not properly exposed yet and the C++ implementation will change soon so while the serialization will work, we need to refactor this code.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-8060

[jorisvandenbossche]

Thanks! added some comments

[wesm]

Is there a JIRA about defining a e.g. Flatbuffers-based serialization protocol (probably better than Protobuf so we avoid having to introduce a library dependency) for expressions? Seems like it might provide a more scalable solution for serialization and could eventually succeed the strategy here

[kszucs]

Created JIRA https://issues.apache.org/jira/browse/ARROW-8214

[kszucs]

+1, build failure is unrelated.