Add opt-in digest validation header to detect stale server parameters

[bitcoin-coder-bob]

Summary

GetInfo already returns a digest field (SHA256 of the JSON-marshalled response). Clients should be able to send this digest as a header on subsequent API calls so arkd can detect when they're operating with stale server parameters.

Behavior

• Clients optionally attach an X-Ark-Digest header (or x-ark-digest gRPC metadata) to any API call.
• If arkd sees the header and the digest doesn't match the current one, it returns a structured DIGEST_MISMATCH error (FailedPrecondition, code 48) with the current_digest in metadata, telling
  the client to re-fetch GetInfo.
• If the header is absent, no validation occurs — completely client opt-in.
• GetInfo itself is always exempt from validation (clients need it to obtain the digest).
• Only public service methods (ArkService, IndexerService) are validated — admin/wallet calls are unaffected.

[bitcoin-coder-bob]

working off PR: #958