feat: pluggable snapshot storage broker with per-snapshot signed URLs

[Anish Ramasekar (aramase)]

Adds a vendor-neutral snapshot storage broker. The control plane mints
short-lived, per-snapshot signed URLs and atelet reads and writes snapshots over
plain HTTP, so the node links no cloud SDK and holds no cloud credential. The
cloud-specific signer runs out of process as a sidecar over a Unix socket, which
keeps the control-plane core free of any cloud SDK as well. Ships an S3 sidecar.
Off by default.

This is the storage-decoupling touchpoint from the Cloud Provider Decoupling
design, and it stands up the same per-snapshot capability #777 needs: the node
stops holding a broad, permanent storage credential. It does not yet make the
authorization decision before minting; that needs the broader control-plane
authz layer.

Design doc: https://docs.google.com/document/d/1I2SlYey0Y2mcGQBZNpfYufWhwLbQDmaqI5T56sz33_4/edit?usp=sharing

What's in it

• storagebroker interface and registry in ate-api-server. New(backend)
  selects one; empty disables signing and the existing in-atelet path is
  unchanged.
• ate-api-server pre-mints per-snapshot capabilities at resume (actor plus
  golden) and suspend (write), passed to atelet via a new signed_access map on
  the atelet proto.
• atelet signedObjectStore satisfies the existing ategcs.ObjectStorage and
  speaks S3 (presigned GET, POST starts-with policy) or Azure (PUT plus token)
  over net/http, with no SDK and no credential.
• UDS broker client so the signer is a sidecar. ate-api-server links 0 AWS SDK
  packages (was 39 with the in-tree signer).
• cmd/s3-broker-server: the S3 sidecar (presigned GET per object, one POST
  policy per snapshot).
• Selected via ATE_SNAPSHOT_BROKER (for example uds), separate from atelet's
  ATE_STORAGE_BACKEND.

Testing

• Unit tests for the mint wiring and the signed HTTP read/write dispatch. Full
  verify-all is green.
• Broker-on kind e2e (parking suspend/resume) exercises the sidecar mint and
  atelet presigned GET/POST end to end.
• Benchmarks show the mint is off the byte path: resume p50 is 244 ms native vs
  253 ms in-tree signer vs 243 ms over the socket, so no measurable latency.

Not in this PR

• The authorization decision before minting (the decision half of Per-snapshot authorization #777).
• A GCS broker, and retiring atelet's existing S3 object-store client to take the
  node to zero SDK.
• Chunked upload for very large snapshots.
• Wiring the sidecar into the in-repo manifests so CI exercises the broker path.