Send Content-Type header with cargo publish requests

[dralley]

What does this PR try to resolve?

Sends an appropriate Content-Type header with cargo publish requests.

Resolves:

#16830

How to test and review this PR?

If necessary, find a way to log the request headers / metadata - I tested this against my in-progress registry implementation. Maybe there is logging mode which can skip that step, I'm not sure.

[rustbot]

r? @weihanglo

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The reviewer was selected based on:

• Owners of files modified in this PR: @ehuss, @epage, @weihanglo
• @ehuss, @epage, @weihanglo expanded to ehuss, epage, weihanglo
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[epage]

Note that our contrib guide asks for Issues to be tagged S-accepted before moving onto PRs.

In addition, this would best be handled with separate PRs because

• They are unrelated
• They have different compatibility considerations and on or more might need team sign off with an FCP

[dralley]

Ok, I can split into two PRs. Apologies.

And I will wait before filing a PR for the other issue.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[weihanglo]

#16830 (comment)

Would you mind also updating docs?

I guess we can instead specify content-type in each API respectively