demo-apps: nats and pubsub

[andrussal]

This PR adds the second set of example applications to logos-blockchain-testing:

• nats
• pubsub

These examples extend the set beyond the baseline clustered-app shape. They keep the same overall model, but apply it to less simple runtime and client behavior.

Review note

These example PRs do not need critical review. The goal is to get useful example integrations into the repo and improve them from there.

What this PR covers

• nats covers an external service with a multi-port runtime and a non-HTTP client path
• pubsub covers websocket roundtrip and reconnect workloads, including k8s

This PR shows that the same TF model still works once the application is no longer just a simple clustered service.

Integration shape

Each app is split into four parts:

• App or service — the code under test
• TF integration — the code that connects the app to TF
• Workloads and expectations — reusable behavior and checks
• Example binaries — small programs that run one sample scenario