Add support for exclusion constraints in create_table operation

[kvch]

This PR adds a new type of constraint to create_table operation called exclude to support exclusion constraints.

New options:

• exclude.index_method: name of the index method (e.g. btree, gist, etc.)
• exclude.elements: exlusion elements
• exclude.predicate: WHERE clause for partial indices

Example constraint forbidding overlap in room reservations:

{
  "name": "52_create_table_with_table_exclusion_constraint",
  "operations": [
    {
      "create_table": {
        "name": "room_reservations",
        "columns": [
          {
            "name": "id",
            "type": "serial"
          },
          {
            "name": "start",
            "type": "timestamp"
          },
          {
            "name": "end",
            "type": "timestamp"
          },
          {
            "name": "canceled",
            "type": "boolean",
            "default": "false"
          }
        ],
        "constraints": [
          {
            "name": "forbid_double_booking",
            "type": "exclude",
            "exclude": {
              "index_method": "gist",
              "elements": "id WITH =, tsrange(\"start\", \"end\") WITH &&",
              "predicate": "NOT canceled"
            }
          }
        ]
      }
    }
  ]
}

The tests I added require the extension btree_gist. So I added it for make examples and running go test.

[andrew-farries]

Looks good 👍

I wonder whether we can do without the need for the btree_gist extension though.

[kvch]

I think the simple test provides enough coverage already. I am happy to remove the more complex tests. \o/