Implement AutoMerge option to AcceptMergeRequest

[jakubbortlik]

Feature Description

The new go library implements the auto-merge option for AcceptMergeRequest. I'd like to have this implemented in gitlab.nvim too so I don't have to go to the website to click the "Set Auto-Merge" button or I don't have to rely on glab mr merge --auto-merge.

[harrisoncramer]

Blocked by upgrading the core Go library -- could be an opt-in thing but we'd have to merge that first. Likely we would have to find a way to gracefully migrate users to the newer Go versions + packages, as the binary doesn't ship with the plugin right now.