Draft: Apply etag caching to pipeline status in pipeline editor

What does this MR do?

For #326085 (closed).

This updates the polling implementation for the pipeline status in the pipeline editor to use etag caching. This MR in particular does the following:

• Pass the initial pipeline etag resource path to Vue. This is then saved in apollo cache so it can be updated later.
• Update the resource path in apollo when the user creates a new commit in the pipeline editor. Because a new pipeline is triggered, the resource path will change. The new path is provided in the CreateCommit mutation response.

Screenshots

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

Before	After
Normal Polling

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