Expose ETag header of API requests to XHR requests

What does this MR do and why?

Contributes to #479248 (closed)

Problem

Access-Control-Expose-Headers header is missing the ETag which prevents clients from using caching mechanisms.

Solution

Explicitly allow Etag header.

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Before	After

How to set up and validate locally

1. curl -I --header 'Origin: http://localhost:8080' http://gdk.test:3000/api/v4/projects/2/repository/tree
2. You should see ETag in Access-Control-Expose-Headers header