refactor: move handler to go-lib package

[marcelosousa]

Description

In an effort of Reviewpad to become code host agnostic, we should not have the handler package as part of Reviewpad anymore as it is not actually directly used by the core package.

Proposed solution

Move the handler package to the go-lib (closed-source) library.

[reviewpad]

AI-Generated Summary: This issue describes the need to make Reviewpad code host agnostic by moving the handler package to the go-lib (closed-source) library, as it is not directly used by the core package.