Add support for creating foreign key constraints using create_constraint

[kvch]

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

Examples

Foreign key

{
  "name": "44_add_foreign_key_table_reference_constraint",
  "operations": [
    {
      "create_constraint": {
        "type": "foreign_key",
        "table": "tickets",
        "name": "fk_sellers",
        "columns": [
          "sellers_name",
          "sellers_zip"
        ],
        "references": {
          "table": "sellers",
          "columns": [
            "name",
            "zip"
          ],
          "on_delete": "CASCADE"
        },
        "up": {
          "sellers_name": "sellers_name",
          "sellers_zip": "sellers_zip"
        },
        "down": {
          "sellers_name": "sellers_name",
          "sellers_zip": "sellers_zip"
        }
      }
    }
  ]
}

Closes #81

[andrew-farries]

Multi-column FKs currently can't be duplicated due to #475.

Once that is resolved we can implement duplication of multi-column FKs on this PR, which is the last addition that needs to happen here before merge.