Add user identity (enduser.id) attribute to OpenTelemetry

### Feature Description

Currently, the OpenTelemetry (OTel) traces and spans generated during AgentCore invocation (`InvokeAgentRuntime` / `gen_ai` execution) capture telemetry metrics and session identifiers, but do not consistently populate user identity in the span attributes.

We request adding support to automatically populate user identity (or allow passing custom user identity metadata) onto OTel span attributes, following the OpenTelemetry Semantic Conventions for user attributes (`enduser.id`).

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### Use Case & Impact

1. **User-level Observability & Debugging**: When troubleshooting agent execution failures or slow agent runs in observability platforms (e.g., Datadog, Grafana, Dynatrace, OpenSearch), SREs and developers need to trace issues back to specific end-users.
2. **Auditability & Compliance**: Enterprise compliance standards often require linking LLM invocations and tool calls directly to the initiating user identity.
3. **Usage Attribution & Quotas**: Facilitates per-user telemetry tracking, token usage analytics, and user-level rate limiting.

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### Proposed Solution

- **Standard Attribute**: Set the span attribute `enduser.id` on root invocation spans and downstream tool/model execution spans according to the [OpenTelemetry Semantic Conventions for User Attributes](https://opentelemetry.io/docs/specs/semconv/general/attributes/#user-attributes).
- **Configuration / Propagation**:
- Extract the user ID automatically from Inbound Authentication JWT tokens (e.g., Cognito `sub` or custom claim) if passed via AgentCore Identity/Auth headers.
- Expose a mechanism in `BedrockAgentCoreApp` or request context context/metadata to explicitly inject `user_id` when initializing or handling incoming requests.

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### Example / Expected Behavior

When an agent invocation span is generated, the resulting OpenTelemetry span attributes should include:

```json
{
"gen_ai.system": "aws.bedrock",
"enduser.id": "usr_987654321",
"session.id": "session_12345"
}

Inizia tracciando la creazione degli span OpenTelemetry per InvokeAgentRuntime e l’esecuzione di gen_ai, quindi esamina la gestione delle requests di BedrockAgentCoreApp e gli header Identity/Auth di AgentCore. Confronta il contesto di identità disponibile con la convenzione OpenTelemetry enduser.id; il lavoro è completato quando gli span root e downstream espongono in modo coerente l’identità richiesta, inclusi i metadati utente forniti esplicitamente.

Scritto dal modello di indicizzazione a partire dal testo della issue.