[YSQL] Enable online DDL for ALTER COLUMN SET n_distinct metadata change

[ddorian]

Jira Link: DB-6052

Description

I think making the change below is just a metadata change and should be supported by us:

CREATE TABLE public.event_search (
    event_id text,
    room_id text,
    sender text,
    key text,
    vector tsvector,
    origin_server_ts bigint,
    stream_ordering bigint
);
ALTER TABLE ONLY public.event_search ALTER COLUMN room_id SET (n_distinct=-0.01);

Reported in #1124 (comment) @tverona1

Warning: Please confirm that this issue does not contain any sensitive information

• I confirm this issue does not contain any sensitive information.