Column creation and copying should use attnum

[mathemancer]

Problem

Currently, most column creation and copying operations are using name or index to identify columns. This is brittle, and we've decided to change to using the attnum property of columns instead.

Proposed solution

Each function in db/columns/operations/create.py should use attnum instead of column_index, and should properly use the table_oid, attnum pair to identify columns for its logic. Use the function get_column_name_from_attnum function in db.columns.operations.select for this where needed.

Additional context

We'll clearly need to modify any place where the functions in the above file are called as well.

Beware, some internal functions in the copying logic will need to be changed, and some have misleading variable names (e.g., _duplicate_column_data). We may need to connect some pieces with logic to get index from attnum where other code we're not changing is called.