Add exclusion constraints to analyze/read_schema

[kvch]

This PR adds support for displaying exclusion constraints in the output of analyze/read_schema.

Attributes:

• name: name of the constraint
• method: selected index method (exclude.index_method option)
• columns: the list of columns included in the constraint
• predicate: conditional expression if partials index
• definition: constraint definition

go run . analyze
{
...
  "excludeConstraints": {
    "forbid_duplicated_titles": {
      "name": "forbid_duplicated_titles",
      "method": "btree",
      "predicate": "(title IS NOT NULL)",
      "columns": [
        "title"
      ],
      "definition": "EXCLUDE USING btree (title WITH =) WHERE ((title IS NOT NULL))"
    }
  },
...
}

Furthermore, I added a new attribute called exclusion to indexes to show if the index belongs to an exclusion constraint.

[kvch]

Renamed to Method so it is consistent with Method in indexes.