Rename factory to handler

[ibraheemdev]

PR Type

Refactor

PR Checklist

• Documentation comments have been added / updated.
• A changelog entry has been made for the appropriate packages.
• Format code with the latest stable rustfmt

Overview

This PR renames the Factory trait to Handler, and the Handler struct to HandlerService. This change better reflects the purpose of these types.

The trait Factory is not satisfied is a common error that has caused a lot of confusion as it is pretty cryptic. The trait Handler is not satisfied makes more sense. I also added documentation listing the requirements of a valid handler.

This is a breaking change (even users aren't really meant to implement Factory manually), but is important for ergonomics.

I also removed the third type parameter (O) from Handler which simplifies the type definition overall.

[robjtede]

Add changelog entry.

Rename Handler to HandlerService and rename Factory to Handler.