refactor(output-datatypes): write down the desired behavior for the various output formats

[cpcloud]

We have a lot of issues related to converting execution output to Pandas.

This issue is to track a Ibis-specific specification for how Ibis types map to Pandas types as well as details on edge case handling.

[kszucs]

I think the ideal use case would be to have a well-defined behavior for ibis => pyarrow and rely on pyarrow => pandas to specify ibis => pandas expectations. I successfully use this approach in the chDB backend:

• #8497