Add Entity and PurgeableResource protocols to use with the Annotated types

[jptrindade]

These protocols are to be used with the implemented Annotated types to represent these inmanta classes on python.
Make sure to add the appropriate attributes for these protocols.

Example usage:

Entity(Protocol):
    requires: list
    provides: list

MyCustomEntityProtocol(Entity):
   value: int

MyCustomEntity: typing.TypeAlias = typing.Annotated[MyCustomEntityProtocol, ModelType["std::MyCustomEntity"]]

[sanderr]

Notes / suggestions:

1. requires: Sequence[Entity] and similar
2. Not 100% certain, but I think subprotocols still need to inherit from Protocol directly
3. Consider whether PurgeableResource protocol belongs in inmanta.plugins or in inmanta_plugins.std