Add trust quorum proxy endpoints and use serde_with for hex encoding

Add three new proxy endpoints to the sled-agent API that allow forwarding
trust quorum operations to other nodes:

- POST `/trust-quorum/proxy/commit`
- POST `/trust-quorum/proxy/prepare-and-commit`
- POST `/trust-quorum/proxy/status`

Also refactors the trust quorum types to use `serde_with` with `Hex` encoding
for cleaner serialization of digests and encrypted rack secrets, replacing
manual hex string parsing with automatic encoding/decoding.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: plaidfinch