python/sedonadb: Make explain plans easier to programatically inspect

[paleolimbot]

In #733 and a few other places (I think Parquet pruning) we use the explain output to assert that something is or isn't happening in the internals of the engine. We currently don't have a great way to inspect these.

We should probably have .explain() return an object like:

class ExplainPlan:

    def __init__(self, df, options):
        pass

    def __repr__(self):
        # Current output or close
        pass

...to which we could add methods to more robustly inspect the structure/metrics.