No foreign key support on template based partitioning

[Asafb26]

pg_partman has a crucial solution for cases that partition key is not a part of the primary key, provided as a template table to add a PK on sub tables but not on the partition table.
The issue is that now I cannot point to the partition table with a FK constraint, there is no PK on that table and I get an error: ERROR: there is no unique constraint matching given keys for referenced table

See discussion here with reproducible use-case #638