Add support for merge_trains_skip_train_allowed to project resource/datasource

This change adds a new configuration option called "merge trains skip train allowed" to GitLab project settings. This feature allows merge requests in a merge train to be merged immediately without waiting for their automated tests (pipelines) to complete first. The change updates both the data source (for reading project settings) and the resource (for managing project settings) to support this new option. It also includes test updates to verify the feature works correctly. This gives project administrators more flexibility in how they handle merge trains when they want to prioritize speed over waiting for all automated checks to finish.

Fixes #6352 (closed)