Add a drop_multicolumn_constraint operation

[andrew-farries]

Add a new drop_multicolumn_constraint operation type. The new operation is used to drop multi-column (and single-column) CHECK, UNIQUE, and FOREIGN KEY constraints:

The following migration drops a multi-column constraint called check_zip_name, and defines up and down SQL data migrations for each column covered by the constraint:

{
  "name": "48_drop_tickets_check",
  "operations": [
    {
      "drop_multicolumn_constraint": {
        "table": "tickets",
        "name": "check_zip_name",
        "up": {
          "sellers_name": "sellers_name",
          "sellers_zip": "sellers_zip"
        },
        "down": {
          "sellers_name": "sellers_name",
          "sellers_zip": "sellers_zip"
        }
      }
    }
  ]
}

• On operation Start each column covered by the constraint is duplicated, ignoring the constraint to be dropped. Triggers for data migrations are created for each column covered by the constraint using the SQL for each column from the migration file.
• On Complete, the duplicated column is renamed to the original name and triggers are removed.
• Validate ensures that all columns covered by the constraint have data migrations defined, and only columns covered by the constraint have data migrations defined.

The new operation is in addition to the existing drop_constraint operation which only drops single column constraints. This old operation type is preserved for backwards compatibility but will be removed as a breaking change before a v1 release.

#489 tracks the removal of the drop_constraint operation.