Implement AttributeVector

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`.

Create an `Attribute_T` type var bound to `Attribute` and use as the Generic type for `AttributeVector`. This typing will not encapsulate access mode or datatype to ensure correct use because this would be difficult to define properly. Instead users will just get what they pass in. For example, `AttributeVector({1: AttrR(Int()), 2: AttrW(String())})` would not provide any helpful type safety when accessing the vector, so this should not be done.