[INTPYTHON-TODO]: PoC Expression Conversion Abstraction

[Jibola]

Summary

This PR introduces an abstraction to convert some of our simple $expr queries to $match queries without $expr in them. If the $expr query cannot be converted to a $match query, it will leave it as a $expr. This behavior is nested. Currently this is limited to $eq and $in operations for now, but in the future can be extended to other $expr conversions.

This only affects the querying done in self.match_mql as the querying for lookups, groupings, or aggregation pipelines are out of scope for this pr.

Changes in this PR

• Introduce the QueryOptimizer abstraction (which can be changed; it doesn't store state so it does not need to remain an object)
• Introduced ExpressionConverters which house the logic for converting queries from $expr usage to their $match equivalents.
• Added several tests to confirm the behavior of the $eq, $in, $and, $or in drilling down of expression conversion.

Test Plan

• Manual testing through python manage.py shell See an example query below.
• Added test cases. Some old tests will fail and need to be fixed

Screenshots (grabbed text)

# Before
>>> Author.objects.filter(author_city__in=["London"])
{
    "$match": {
        "$expr": {
            "$and": [
                "$in": ["$author_city", "London"]
            ]
        }
    }
}

# After 
>>> Author.objects.filter(author_city__in=["London"])
{
    "$match": {
        "$and": {
            "$in": ["$author_city", "London"]
        }
    }
}

Callouts

• This is still a proof of concept. Like said above, QueryOptimizer should be removed and left as its functions if it is not going to retain state. Initialization is unnecessary.
• This will not work on Embedded Models until a$getField converter is made.