bug(SQLAlchemy): Handle edge case get-field and set-field operators

[Sambigeara]

In an earlier version of Cerbos, certain inputs could lead to complex query plan outputs. A full example can be seen in this issue.

As it stands, the SQLAlchemy adapter only handles a finite number of operators: and, or and not. In some query plans, we see other operators: get-field and set-field. These are currently unsupported and break the get_query function.

In general, I think we try to avoid those operators altogether, but I still think we should protect against them and allow successful query resolution.

[Sambigeara]

Counter-thought: would we want to output queries generated with complex plans like the above? We wouldn't want to topple the user's database 🤔.