sql2pgroll: Ignore IF NOT EXISTS in CREATE TABLE statement conversion

[andrew-farries]

Convert CREATE TABLE IF NOT EXISTS statements to create_table operations as though the IF NOT EXISTS clause was not present.

A CREATE TABLE statement like this:

CREATE TABLE IF NOT EXISTS "dogs" (
        "id" integer PRIMARY KEY GENERATED ALWAYS AS IDENTITY (sequence name "dogs_id_seq" INCREMENT BY 1 MINVALUE 1 MAXVALUE 2147483647 START WITH 1 CACHE 1),
        "name" varchar(255) NOT NULL,
        "age" integer NOT NULL
);

is now converted to:

{
  "operations": [
    {
      "create_table": {
        "columns": [
          {
            "generated": {
              "identity": {
                "sequence_options": "SEQUENCE NAME dogs_id_seq INCREMENT 1 MINVALUE 1 MAXVALUE 2147483647 START 1 CACHE 1 ",
                "user_specified_values": "ALWAYS"
              }
            },
            "name": "id",
            "pk": true,
            "type": "int"
          },
          {
            "name": "name",
            "type": "varchar(255)"
          },
          {
            "name": "age",
            "type": "int"
          }
        ],
        "name": "dogs"
      }
    }
  ]
}

rather than a raw SQL migration.

See the discussion on #719 for the rationale behind this change.