Add merge related settings for Groups API support

Add support for the following fields in the Groups API:

• only_allow_merge_if_all_discussions_are_resolved
• only_allow_merge_if_pipeline_succeeds
• allow_merge_on_skipped_pipeline

Implementation Guide

• Read the CONTRIBUTING.md guide for setting up your local development environment and clone the community fork of this project.
• Add the three fields to any Group structs used in groups.go. So structs returned when getting groups and structs used to update groups.
• Add tests, ideally also integration tests.
• Run make reviewable before raising an MR.