Update schema during add column validation

[andrew-farries]

Update the schema during add_column operation validation so that validation of subsequent operations in the same migration can see the new column.

This means that migrations that add a new column and then perform some other operation, like adding an index, on that column can be validated because the new column is visible to the create_index operation during it's validation phase.

This means that the following migration is now able to validate:

{
  "name": "43_multiple_ops",
  "operations": [
    {
      "add_column": {
        "table": "players",
        "column": {
          "name": "rating",
          "type": "integer",
          "comment": "hello world",
          "check": {
            "name": "rating_check",
            "constraint": "rating > 0 AND rating < 100"
          },
          "nullable": false
        }
      }
    },
    {
      "create_index": {
        "name": "idx_player_rating",
        "table": "players",
        "columns": [
          "rating"
        ]
      }
    }
  ]
}

Previously, the new column would not have been visible to the create_index operation and its validation would have failed.

This PR does for the add_column operation what #455 did for the create_table operation.

Part of #239