fix(repo): accept gzip-compressed index.yaml responses

[vigneshakaviki]

What this PR does / why we need it:

Fixes #32479.

HTTPGetter currently disables HTTP compression across the board. That is correct for chart archive downloads, but it also means repository index requests never advertise gzip support.

This change keeps chart downloads unchanged and narrowly opts index.yaml requests into gzip by:

• sending Accept-Encoding: gzip only for index.yaml requests
• transparently decompressing gzip responses only for those requests

That allows large repository indexes to be transferred compressed without changing .tgz download behavior.

Special notes for your reviewer:

This is intentionally narrow. Enabling HTTP compression globally in HTTPGetter would risk transparently decompressing chart archives when servers send Content-Encoding: gzip.

Validation run locally:

• go test ./pkg/repo/v1 -run TestDownloadIndexFile/should_accept_gzip-compressed_index_responses -count=1
• go test ./pkg/getter ./pkg/repo/v1 -count=1
• PATH="$(go env GOPATH)/bin:$PATH" make test-style

If applicable:

• this PR contains user facing changes (the docs needed label should be applied if so)
• this PR contains unit tests
• this PR has been tested for backwards compatibility