New property PostgresNode::port_manager

[dmitry-lipetsk]

Our PostgresNode has the RO-property os_ops:

testgres/testgres/node.py

Lines 310 to 314 in 44d61c2

	@property
	def os_ops(self) -> OsOperations:
	assert self._os_ops is not None
	assert isinstance(self._os_ops, OsOperations)
	return self._os_ops

I think we should have the RO-property port_manager, too.