feat: add OpenTelemetry tracing support behind otel feature flag

- Add tracing::instrument spans to executor (workflow.execute),
  host (workflow.start, event.publish, event.process)
- Add otel feature flag to wfe-core and wfe crates
- Add wfe/src/otel.rs helper for OTLP exporter initialization
- Dependencies: tracing-opentelemetry, opentelemetry, opentelemetry_sdk,
  opentelemetry-otlp (all optional behind otel feature)
- Step execution stays at info level, executor internals at debug

Cargo.toml

@@ -18,6 +18,10 @@ chrono = { version = "0.4", features = ["serde"] }
 thiserror = "2"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+tracing-opentelemetry = "0.28"
+opentelemetry = "0.27"
+opentelemetry_sdk = { version = "0.27", features = ["rt-tokio"] }
+opentelemetry-otlp = { version = "0.27", features = ["tonic"] }
 
 # HTTP
 reqwest = { version = "0.12", features = ["json"] }