[copy_from] Refactor ranged requests, fix HTTP source

[ParkMyCar]

Stacked on top of #31144

This PR refactors ranged get requests so the logic for forming the header value is shared for both the HTTP and S3 source. It also fixes a bug where sometimes HEAD requests (which we use to get metadata for a file) are not supported, so we fallback to a GET request but quickly drop the body.

Motivation

Fix a bug in ranged HTTP requests I found in the demo today

Tips for reviewer

review only the final commit, the one titled "start, fix ranged requests"

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[jkosh44]

LGTM, but we should probably add tests with the Range header if we don't already have some.

[ParkMyCar]

LGTM, but we should probably add tests with the Range header if we don't already have some.

Will do!