Rebuild expression tree in Python + export event searching to Python

[ChristopherRabotin]

High level description

#519 introduced expression trees and event reporting to Rust. It's extremely powerful and pretty fast, but the exact same interface won't work as-is in Python with pyo3, cf PyO3/pyo3#4694 (comment) .

Requirements

• Allow Python users to build ScalarExpr in Python
• Allow Python users to search for events with their manually built ScalarExpr

Test plans

1. Build a set of scalar expressions in Python
2. Call the event finder

Design

Rebuild the whole expression management in Python, cf. the comment above. Then implement the From/To trait to convert from one to another. Make sure to allow serde of these scalar expressions using the Rust implementation.