Add a 'set comment' sub-operation to 'alter column'

[andrew-farries]

Allow 'alter column' operations to set the comment on a column:

{
  "name": "02_change_comment",
  "operations": [
    {
      "alter_column": {
        "table": "events",
        "column": "name",
        "comment": "The full name of the event"
      }
    }
  ]
}

This is a versioned migration so the column to which the comment is applied is duplicated and backfilled according to the up and down SQL supplied with the 'alter column' operation. up and down default to a simple copy of the field between new and old columns.

The intention is that this can be combined with a 'change type' sub-operation if there is some column metadata that should be updated as part of the type change:

{
  "name": "35_alter_column_multiple",
  "operations": [
    {
      "alter_column": {
        "table": "events",
        "column": "name",
        "name": "event_name",
        "type": "text",
        "comment": "{type: some-metadata}",
        "up": "name",
        "down": "name"
      }
    }
  ]
}

Fixes #328