[YSQL] Allow adding primary key to partitioned table using ALTER TABLE

[sushantrmishra]

Jira Link: DB-736
Currently adding primary key to partitioned table in YugabyteDB fails. This works fine in PostgreSQL.

CREATE TABLE orders2 (
order_id bigint NOT NULL,
order_date timestamp
) PARTITION BY RANGE (order_date) ;
ALTER TABLE orders2 ADD PRIMARY KEY (order_id,order_date);

ERROR: adding primary key to a partitioned table is not yet implemented