Introduce new add to merge train when checks pass service

app/assets/javascripts/vue_merge_request_widget/constants.js

...cripts/vue_merge_request_widget/stores/mr_widget_store.js

app/models/merge_request.rb

app/services/auto_merge_service.rb

doc/api/graphql/reference/index.md

...javascripts/vue_merge_request_widget/mixins/auto_merge.js

...scripts/vue_merge_request_widget/mixins/ready_to_merge.js

...ascripts/vue_merge_request_widget/stores/get_state_key.js

...cripts/vue_merge_request_widget/stores/mr_widget_store.js

...auto_merge/add_to_merge_train_when_checks_pass_service.rb

...erge/add_to_merge_train_when_pipeline_succeeds_service.rb

ee/app/services/auto_merge/merge_train_service.rb

ee/app/services/ee/auto_merge_service.rb

ee/app/services/ee/system_note_service.rb

ee/app/services/merge_trains/add_merge_request_service.rb

ee/app/services/system_notes/merge_train_service.rb

.../feature_flags/wip/merge_when_checks_pass_merge_train.yml

ee/spec/factories/merge_requests.rb

...rge_trains/user_adds_merge_request_to_merge_train_spec.rb

..._trains/user_adds_to_merge_train_when_checks_pass_spec.rb