Design: `explain` time parameter overrides for explainers

[jklaise]

This issue is to track discussions around what parameters a user should be able to override at explain time regardless of the explainer setup at construction. Initial discussion can be found here: #523 (comment).

To summarize, the line between what parameters we may want to override vs what should be fixed is not always straightforward. There are also considerations wrt alibi API standardization as well as production use cases (e.g. limiting arbitrary overrides).