[#8735] YSQL: Implement dropping primary key

Summary:
In #1104 / D10057 we implemented `ALTER TABLE ADD PRIMARY KEY` command.

This diff extends that work to implement `ALTER TABLE DROP CONSTRAINT pkey`.

We cannot drop primary key of:
* Partitioned tables and table partitions.
* Tables having rules.

Note that just like `ALTER TABLE ADD PRIMARY KEY`, this is designed for database setup and ORM migrations, and does not guarantee data safety when the table is being concurrently modified!

---

Resolves #8735

Test Plan:
ybd --java-test 'org.yb.pgsql.TestPgAlterTableChangePrimaryKey'

`TestPgAlterTableAddPrimaryKey` has been tweaked into `TestPgAlterTableChangePrimaryKey` which tests both adds and drops.

Reviewers: mihnea, tverona, neil, jason, myang, dmitry

Reviewed By: dmitry

Subscribers: yql

Differential Revision: https://phabricator.dev.yugabyte.com/D17939

java/yb-pgsql/src/test/java/org/yb/pgsql/BasePgSQLTest.java

@@ -1476,6 +1476,12 @@ protected boolean isIndexOnlyScan(Statement stmt, String query, String index)
     return doesQueryPlanContainsSubstring(stmt, query, "Index Only Scan using " + index);
   }
 
+  /** Whether or not this select query uses Seq Scan. */
+  protected boolean isSeqScan(Statement stmt, String query)
+      throws SQLException {
+    return doesQueryPlanContainsSubstring(stmt, query, "Seq Scan on");
+  }
+
   /**
    * Whether or not this select query requires filtering by Postgres (i.e. not all
    * conditions can be pushed down to YugaByte).