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`).

---

### 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.

---

### 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.

---

### 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"
}

Start by tracing OpenTelemetry span creation for InvokeAgentRuntime and gen_ai execution, then inspect BedrockAgentCoreApp request handling and AgentCore Identity/Auth headers. Compare the available identity context with the OpenTelemetry enduser.id convention; done means root and downstream spans consistently expose the requested identity, including explicitly supplied user metadata.

Written by the indexing model from the issue text.