Use etag caching for merge request attributes which must be invalidated on project attributes change

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The idea is to gradually migrate the fields out of MergeRequestPollWidgetEntity to the MergeRequestPollCachedWidgetEntity. The following attributes must be invalidated when project attributes change:

Frondend attribute	Backend attribute	When should be invalidated
createIssueToResolveDiscussionsPath	create_issue_to_resolve_discussions_path	changing issues_access_level of a project (depends on attr individual to a user)
sourceBranchProtected	source_branch_protected	setting a branch to protected
shouldBeRebased	should_be_rebased	changing ff_merge_must_be_possible? project attr
ffOnlyEnabled	ff_only_enabled	changing merge_requests_ff_only_enabled project attr
project_archived	project_archived	changing project archived attr
hasMergeableDiscussionsState	mergeable_discussions_state	changing only_allow_merge_if_all_discussions_are_resolved? attr and discussions iteration
allowCollaboration	allow_collaboration	project and protected branch

Related issue: gitlab-foss#61559 (closed)