Add target_branch and approval_rules to merge_request webhook

This Mr is reopened from !138564 (merged), as it got reverted.

This Mr does a few things:

• it add target_branchand approval_rules to the merge_request webhook payload, and also to the changes field.
• It provides a small refactoring around hook_association_changes, as discussion on a thread on this mr(thanks kerryzor).
• Alphabetically orders lib/gitlab/hook_data/merge_request_builder.rb build, as it was getting too messy.

Related to #434264

Signed-off-by: Lucas Zampieri lzampier@redhat.com