ENH: Make `attributes` field on `PNode` and `PProvisionalNode` mandatory in pytask v0.6.0.

[tobiasraabe]

Is your feature request related to a problem?

In #648, all implementations of PNode and PProvisionalNode received a new field attributes to store arbitrary data on a node like on a task. The field is not yet mandatory because requiring the field on the protocol would potentially break people's code. Instead we display warnings.

The warnings should be removed in v0.6.0 and the field should be added to the protocols.