[ENH] Factor out remaining use of `fqn` in codebase

[charlesbluca]

Is your feature request related to a problem? Please describe. Once https://github.com/dask-contrib/dask-sql/pull/854 is finalized and merged, there will only be one remaining use of context.fqn in the codebase, used to grab the table & schema names for our table scan plugin.

Describe the solution you'd like Not sure if there's an alternative method we've established that we could use to grab this information, which would allow us to remove fqn and probably do some refactoring to DaskTable and related code since we would no longer need to rely on it for schema information.