[remote/downloader] Wire credential helper to repository downloads

[Yannic]

Progress on #16595

[sgowroji]

Hi @Yannic, Could you please fix the above buildkite errors. Thanks!

[thesayyn]

Since credential-helper can set arbitrary headers, how is this going to play out when there's a conflict with existing headers? for instance, bazel sets Accept header, what would be the behavior?

[tjgq]

Since credential-helper can set arbitrary headers, how is this going to play out when there's a conflict with existing headers? for instance, bazel sets Accept header, what would be the behavior?

Both sets of headers will be merged into the request; if there are duplicates, the credential helper wins. (Although I hope this is just a hypothetical example; using Accept as an authorization mechanism sounds like a bad idea.)

(This has now been submitted, see #18173.)

[thesayyn]

using Accept as an authorization mechanism sounds like a bad idea.

why would that be a limitation? Note that there are valid use cases #17829