Add support for creating CHECK constraints with create_constraint

[kvch]

This PR introduces a new constraint type to create_constraint operation called check. Now it is possible to create check constraints on multiple columns.

Example

{
  "name": "45_add_table_check_constraint",
  "operations": [
    {
      "create_constraint": {
        "type": "check",
        "table": "tickets",
        "name": "check_zip_name",
        "columns": [
          "sellers_name",
          "sellers_zip"
        ],
        "check": "sellers_name ~ 'Alice' AND sellers_zip IS NOT NULL",
        "up": {
          "sellers_name": "Alice",
          "sellers_zip": "(SELECT CASE WHEN sellers_zip IS NOT NULL THEN sellers_zip ELSE '00000' END)"
        },
        "down": {
          "sellers_name": "sellers_name",
          "sellers_zip": "sellers_zip"
        }
      }
    }
  ]
}

[kvch]

I've found a bug in column duplication for unique indexes with multiple columns, so the example is failing. I am adding a fix in a separate PR.

[kvch]

Bugfix: #466

[kvch]

It is highly likely we need to adjust the check constraint migrations as well. I will address those issues in a follow-up PR.