Review the usage of `Annotation` type across the app

During https://github.com/hypothesis/client/pull/5658, it was made obvious we need to review how we use the `Annotation` type to represent annotations.

After that, we now have two independent types, the `APIAnnotationData`, which represents an annotation as served/consumed by h API, and `ClientAnnotationData`, which represents the extra attributes added by the client for internal logic.

Then we have the main `Annotation` type which is a combination of both.

For simplicity and historical reasons, the `Annotation` one has been used everywhere, but we need to make a pass over the code and try to be as granular as possible, making sure we narrow down types to `APIAnnotationData` or `ClietnAnnotationData` if applicable.