Support multiple backfill-requiring changes to the same column in a single migration

[andrew-farries]

A migration that contains multiple backfill-requiring operations on the same column will currently fail.

Fixing this is probably a large change to how pgroll works - currently operations are largely independent of one another. To make this work, column duplication and trigger creation would have to move out of the individual operations happen after all operations have Started.

Reproduction

Create a table:

{
  "name": "01_create_table",
  "operations": [
    {
      "create_table": {
        "name": "items",
        "columns": [
          {
            "name": "id",
            "type": "serial",
            "pk": true
          },
          {
            "name": "name",
            "type": "text",
            "check": {
              "name": "name_not_empty",
              "constraint": "name <> ''"
            }
          }
        ]
      }
    }
  ]
}

Then run a migration that removes a constraint from the name column and sets its nullability:

{
  "name": "14_multiple_ops",
  "operations": [
    {
      "drop_multicolumn_constraint": {
        "table": "items",
        "name": "name_not_empty",
        "down": {
          "name": "name"
        }
      }
    },
    {
      "alter_column": {
        "table": "items",
        "column": "name",
        "nullable": true,
        "up": "name",
        "down": "name"
      }
    }
  ]
}

The migration fails because both operations attempt to duplicate the name column.