Add functions to convert table types and/or distribution column of distributed tables

While https://github.com/citusdata/citus/pull/3913 implemented, we had some discussions around the need to these kinds of UDFs. Opening the issue to track. The things that I remember are mostly:

- a UDF to change table type -- name is arbitrary: SELECT convert_table('table_1', to:'reference'); or `SELECT convert_table('table_2', to:'dist', dist_key:=`user_id`)`
- The same UDF or potentially another: `SELECT change_distribution_column('table_1', new_dist_key='event_id')`

We can currently do this via `INSERT INTO .. SELECT` with repartitioning in a scalable way. However, `undistribute_table ` is superior as it creates the views, partitioning hierarchy, indexes and sequences. The goal is to provide something similar.