Add compiler plugin support for append and appendNulls #2124

We have no compiler plugin support yet for append and appendNulls.

append and appendNulls can change the schema of the dataframe (make the type of a value column nullable, lose a schema when working with FrameColumn, etc). Therefore, we need to support these operations by refining the result schema.

After these changes are made, the corresponding notes in the KDocs (AppendingNullsWidensColumnTypes) and website docs should be adjusted or deleted.