Use etag caching for merge request attributes related to pipelines

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The idea is to gradually migrate the fields out of MergeRequestPollWidgetEntity to the MergeRequestPollCachedWidgetEntity. The following attributes are related to merge request pipelines, so it would be more reliable if someone responsible for devopsverify could address this issue:

Frondend attribute	Backend attribute	Depends on the fields
mergePipeline	merge_pipeline	pipeline
pipeline	pipeline/actual_head_pipeline	pipeline
mergeTrainIndex	merge_train_index	pipeline (doesn't exist?)
ciStatus	ci_status	pipeline
hasCI	has_ci	pipeline and ci_service
availableAutoMergeStrategies	available_auto_merge_strategies	pipeline
mergePipelinesEnabled	merge_pipelines_enabled	pipeline
autoMergeStrategy	auto_merge_strategy	pipeline
onlyAllowMergeIfPipelineSucceeds	only_allow_merge_if_pipeline_succeeds	pipeline
autoMergeEnabled	auto_merge_enabled	pipeline
testResultsPath	test_reports_path	pipeline

Related issue: gitlab-foss#61559 (closed)