Add support for `nulls_not_distinct` option in `create_constraint` and `create_index` operations

[kvch]

Add support for nulls_not_distinct option in create_constraint and create_index operations for unique constraints and indexes.

NULLS NOT DISTINCT was introduced in PG 15, so I added some conditionals to check if we can retrieve the column from pg_index. Without this function, we get an exception because indnullsnotdisctict does not exist in PG 14.

Closes #730

[kvch]

I am converting this to draft for now because we need to rework backfilling process significantly to get this working.

[kvch]

Closing because it is outdated.