Allow the add column operation to create foreign key columns.

An example of such an operation is:

{
  "name": "17_add_rating_column",
  "operations": [
    {
      "add_column": {
        "table": "orders",
        "column": {
          "name": "user_id",
          "type": "integer",
          "references": {
            "table": "users",
            "column": "id",
          }
        }
      }
    }
  ]
}

Most of the work to support the operation is in #79.

• The constraint is added on Start (named according to the temporary name of the new column).
• The entire new column, including the foreign key constraint, is removed on Rollback.
• The constraint is renamed to use the final name of the new column on Complete.

Test cases are included for both nullable and non-nullable FKs.