Enable filtering issues by weight via the API

Problem to solve

Currently we can filter issues on many fields as outlined here: https://docs.gitlab.com/ee/api/issues.html#list-issues, but although we have weight support in the API, it isnt supported as a filtering property.

This issue would be to add this filtering ability (and update the list in the docs since product_id is missing from that list as well)

Proposal

Add weight filtering to the Issues API