Etag header support for index.yaml

[illyaMs]

• What are you trying to do?
  Download a list of all charts (download /repository/helm/index.yaml file).
  There seems to be no ETag header support => file is fairly re-downloaded every time (instead of returning a 304 response code if current Etag matches the one provided by client)

• What feature or behavior is this required for?
  This may be very useful for CI automation tools: they may cache the index.yaml file on their side and only do re-download this file when there're changes. Especially may make sense for big helm repositories, where the index.yaml file size is significant.

• How could we solve this issue? (Not knowing is okay!)

1. Send ETag header within a /repository/helm/iindex.yaml response
2. Check Etag header on requests: return 304 status response with no body if request's Etag matches the current one; return 200 status code with complete contents in body otherwise.

[mlukaretkyi]

Hi, we are moving helm source code to nexus public. This github page will be archived. Your issue https://issues.sonatype.org/browse/NEXUS-25202