[YSQL] ALTER TABLE ADD COLUMN ... UNIQUE doesn't work

[frozenspider]

Jira Link: DB-1814

postgres=# CREATE TABLE test(id int PRIMARY KEY);
CREATE TABLE

postgres=# ALTER TABLE test ADD COLUMN user_id BIGINT NULL UNIQUE;
ERROR:  This ALTER TABLE command is not yet supported.

postgres=# ALTER TABLE test ADD COLUMN user_id BIGINT NULL;
ALTER TABLE

postgres=# CREATE UNIQUE INDEX ON test (user_id);
CREATE INDEX

postgres=# \d test
                Table "public.test"
 Column  |  Type   | Collation | Nullable | Default
---------+---------+-----------+----------+---------
 id      | integer |           | not null |
 user_id | bigint  |           |          |
Indexes:
    "test_pkey" PRIMARY KEY, lsm (id HASH)
    "test_user_id_idx" UNIQUE, lsm (user_id HASH)

This is surprising given that we support both adding columns and creating unique indexes when they're issued separately.

[frozenspider]

Simplest case (nullable, no defaults, no CHECK constraints, no FKs) was implemented in ce46d1a, but the general case is still NYI - we need to be able to rollback DocDB DDL changes for that.