[webhooks] Decompress request bodies

[ParkMyCar]

This PR adds support for compressed webhook bodies using the decompression layers in tower_http. It also adds a test to ensure the decompression actually works.

Motivation

More advance support for webhook bodies.

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.

[teskje]

Thanks @ParkMyCar! I'll take over getting this merged if you don't mind.

[def-]

Rebased to deal with Hetzner not having aarch64 availability (to pick my improved detection and AWS fallback): https://buildkite.com/materialize/test/builds/104777

@teskje Oops, I think you have to reopen this yourself:

Checking whether GitHub user ParkMyCar is a member of the code-authorized team

[teskje]

Manual rebuild still works!