Restrict or officially support process inputs storing data in process node attributes

[sphuber]

In #5801 a new keyword is_metadata was added to the InputPort constructor. By default it is False, but when set to True, it signals that inputs to this port will not be linked up as Data nodes to the process node, but the data will be stored on the process node itself, somehow. The port is currently only used internally, by all the ports in the metadata namespace, and the data is mostly stored in the node's attributes, with some exceptions, like the label and description that are stored in columns of the database model.

Currently, the feature is not officially documented. We should decide whether we should allow plugins to add is_metadata ports and have that data automatically stored in the node's attribute, or whether this should be disallowed, and any added ports outside of the AiiDA base classes, will raise an error.