Set relation_column_encoding to AUTO by default

[thomas-vogels]

Description

User-visible Changes

This changes the default of the relation_column_encoding setting from ON to AUTO. This means that Arthur will pick appropriate encodings for columns. (The encoding value is picked based on the type and whether it's used in distribution or sort keys.) Selecting the encoding speeds up loading data (COMPUPDATE can be left off).

Links

Closes #229

Testing

You can check the active setting using:

arthur.py settings arthur_settings.redshift.relation_column_encoding

You can check which encoding is used with show_ddl on a table. Unless you already had the auto setting active, you would see for example columns like this:

    "created_at" TIMESTAMP WITHOUT TIME ZONE NOT NULL,

but then would see an encoding with the new default:

    "created_at" TIMESTAMP WITHOUT TIME ZONE ENCODE zstd NOT NULL,

Deploy Notes

If the relation_column_encoding is set to AUTO in local configurations, then this can be removed now. If this mode is not desired, then relation_column_encoding must be set to OFF now.

Harry's internal note: This has been effectively in place in production given that we have a file in the object store with this content:

{
  "arthur_settings": {
    "redshift": {
       "relation_column_encoding": "AUTO"
    }
  }
}

(This file should now be removed.)

[ynaim94]

LGTM