Add etag caching for pipeline status in pipeline editor

What does this MR do?

For issue #324628 (closed). We're update the polling implementation for the pipeline status in the Pipeline Editor to use etag caching.

This is the frontend change for the etag caching. It should be paired with the backend changes in !58931 (merged) for proper testing.

For frontend changes, the initial ETag path for the pipeline triggered by the latest commit is passed to the vue app and saved in apollo cache. This is updated whenever the the user creates a new commit and triggers a new pipeline.

Screenshots

Instead of getting a bunch of 200s when getPipeline is polled, we should now see a bunch of 304s while the pipeline status hasn't changed.

Before	After
Normal Polling	Cached Responses

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Related to #324628 (closed)