Change the create_index operation to take the columns on which the index should be defined as an list rather than a map.

This ensures that the ordering of the columns in in the index is fixed to the same order as they appear in the migration file.

old format

# foo
operations:
  - create_index:
      name: some-index
      table: tickets
      columns:
        column1: {}
        column2: {}

new format

operations:
- create_index:
    name: some-index
    table: tickets
    columns:
    - column: column1
      # options
    - column: column2
      # options

The changes in this PR are taken from reve-ai#5 🙏

⚠️ This is a breaking change to the migration format ⚠️

We also modify the pgroll update command in this PR so that operations in the old format can be migrated to the new format (although for multi-column indexes the user will still have to ensure that the column order in the updated migration is what they expected).

Fixes #1001