Anaylze schema report should issue a warning on tables without primary keys

[ssherwood]

In general, we recommend that all tables have primary keys in YugabyteDB.

It would be useful if the analyze-schema report would highlight any tables in the migration that do not have a primary key so it can be evaluated and triaged prior to migration.

e.g.

create table baz(
  id int default null,
  created_on datetime default null,
  created_by varchar(40)
)