Add constraints option to create_table and unique constraint support

[kvch]

This PR adds support for a new option of create_table operation named constraints.
It expects a list of constraints that is defined on the table when the table is created.

At the moment the only constraint we support is unique. But it includes support for all options
of unique constraints including NULLS NOT DISTINCT, index configuration settings, constraint deference, etc. Once I am done with these table constraints, I will open a follow-up PR to extend the constraint options for column constraints and create_constraint operation.

Example migration:

{
  "name": "50_create_table_with_table_constraint",
  "operations": [
    {
      "create_table": {
        "name": "phonebook",
        "columns": [
          {
            "name": "id",
            "type": "serial",
            "pk": true
          },
          {
            "name": "name",
            "type": "varchar(255)"
          },
          {
            "name": "city",
            "type": "varchar(255)"
          },
          {
            "name": "phone",
            "type": "varchar(255)"
          }
        ],
        "constraints": [
          {
            "name": "unique_numbers",
            "type": "unique",
            "columns": ["phone"],
            "index_parameters": {
              "include_columns": ["name"]
            }
          }
        ]
      }
    }
  ]
}

The table definition above turns into this table in PostgreSQL:

postgres=# \d phonebook
                                    Table "public.phonebook"
 Column |          Type          | Collation | Nullable |                Default
--------+------------------------+-----------+----------+---------------------------------------
 id     | integer                |           | not null | nextval('phonebook_id_seq'::regclass)
 name   | character varying(255) |           | not null |
 city   | character varying(255) |           | not null |
 phone  | character varying(255) |           | not null |
Indexes:
    "phonebook_pkey" PRIMARY KEY, btree (id)
    "unique_numbers" UNIQUE CONSTRAINT, btree (phone) INCLUDE (name)

Part of #580

[andrew-farries]

This looks good 👍 I left some comments.

I haven't had time to review the sql2pgroll part of the changes yet, will review that as soon as I can.

Once I am done with these table constraints, I will open a follow-up PR to extend the constraint options for column constraints and create_constraint operation.

Thanks, I was going to ask about potential duplication between the Constraint type added in this behaviour and the kinds of constraints that are possible via OpAlterColumn.

[andrew-farries]

optional: we could use strings.Builder and pass it to the addIndexParameters and addDeferrable methods to avoid all the string concatenation.

[kvch]

This might be updated because I have another example in a PR....