Update visitor function to visit temporal system nexus payloads

[spkane31]

What changed?

• Added generic support for marked system payload envelopes in proxy.VisitPayloads.
• Introduced the reserved metadata key __temporal_system_payload.
• When the marker is "true", the visitor decodes the binary protobuf envelope using messageType, visits nested user payloads, and reserializes it without passing the outer envelope to the caller’s codec visitor.
• Added test coverage.

Why?
Handling system nexus messages based on explicit payload metadata makes traversal generic and avoids service, endpoint, operation, or event-correlation registries.

How did you test it?

• Added unit tests confirming nested Payload and Payloads mutations survive envelope re-serialization.
• Added tests for missing/unknown messageType and unsupported encoding.
• Added a WorkflowService gRPC test covering outbound workflow-task completion and inbound workflow-task polling, verifying only nested payloads reach codec visitors.