Add patroni role

Summary

Going forward, in 14.0 we want to be able to have use the postgres_role for database node configurations that aren't necessarily in an HA setup.

Proposal

Towards that goal we are giving patroni its own role rather than including it in the postgres_role. The postgres_role will continue to enable repmgr by default until 14.0, where we are removing repmgr.

References

Split from #5850 (closed) which we can't complete until 14.0