chore: add types for `compute` and `startup` mixins

[erhant]

The shard mixins require type declarations, otherwise we get many IDE errors.

[erhant]

A nice solution to this seems to be adding an shard/attrib.py file with a class that ONLY has the attributes, from which others inherit. This way, we ensure types are same all accross. Some bugs were discovered during this task as well, all related to typings.