Support for ON UPDATE/DELETE SET DEFAULT

[fulghum]

Doltgres supports adding constraints through ALTER TABLE, but the ON DELETE and ON UPDATE clauses don't support the SET DEFAULT option. Dolt supports parsing the ON DELETE/UPDATE SET DEFAULT syntax, but GMS does not implement the logic for it.

CREATE TABLE public.hn_stories (
    id integer NOT NULL,
    title text NOT NULL,
    date timestamp with time zone NOT NULL,
    score numeric NOT NULL,
    hn_username text,
    website_url text
);

CREATE TABLE public.websites (
    url text NOT NULL,
    title text,
    description text,
    twitter_username text,
    github_username text
);

ALTER TABLE ONLY public.hn_stories
    ADD CONSTRAINT hn_stories_website_url_fkey FOREIGN KEY (website_url) REFERENCES public.websites(url) ON UPDATE CASCADE ON DELETE SET DEFAULT;

Discovered while testing Hasura dump loading