Implement AttributeVector

[GDYendell]

Create AttributeVector class similar to ControllerVector, but storing a set of Attribute by non-contiguous index. This will map onto DeviceVector[Signal] in ophyd-async. In contract to ControllerVector in that it does not implement Attribute, it is purely a Mapping[int, Attribute] for convenience.

As AttributeVector is not Attribute, there will have to be a BaseController.add_attribute_vector and add a case for it in __setattr__. Then the creation of the Controller_API should unpack these into the flat list of Attribute for transports to use.

AttributeVector should be generic on and Attribute_T, which can be AttrR, AttrW, or AttrRW (not Attribute), so that vector[0] returns the specific type. I think it won't be possible for the type checker to infer the DataType of the Attribute, though. This isn't possible:

class AttributeVector(Generic[Attribute_T, DataType_T]):

    def __getitem__(self, key: int) -> Attribute_T[DataType_T]:
        ...

it could only do this:

class AttributeVector(Generic[DataType_T]):

    def __getitem__(self, key: int) -> Attribute[DataType_T]:
        ...

and then you can't match on AttrR etc., you have to match on Attribute(access_mode="r").