Adds a generic passthrough path through the API gateway for services
that are not part of the core TrustGraph platform. Enterprise services
(e.g. attestation engine) can communicate through the gateway using
a `thru/` kind prefix without requiring core to know about them.

Gateway changes:
- PassthroughRequest/PassthroughResponse schema types with opaque
  JSON payload forwarding
- PassthroughRequestor ServiceRequestor subclass that wraps/unwraps
  the payload without interpretation
- Route for /api/v1/flow/{flow}/service/thru/{kind} registered before
  the general {kind} route so aiohttp matches multi-segment paths
- Dynamic registry lookup returns AUTHENTICATED flow-level Operation
  for any flow-service:thru/* name, allowing the Mux WebSocket path
  to resolve passthrough services without static registration
- Dispatch manager creates PassthroughRequestor for any kind starting
  with thru/, slotting in between request_response and sender
  dispatcher lookups