[Enhancement Request] Add ECS `agent.*` fields to Elastic Serverless Forwarder

## Use Case

As the administrator of our Elastic Cloud environment, I need to be able to determine which agent version is running in different environments so that I can be certain that that any issues which have been resolved in the agent aren't impacting my users.

## Describe the Enhancement

The Elastic Common Schema (ECS) provides the root-level `agent.*` object to provide information about the agent. Elastic Agent utilizes this object to provide information including agent type, id, name, and version. Please add at least the following fields to the `agent.*` object in Elastic Serverless Forwarder:

| Field | Suggested Value |
|:-----|:-----|
| `agent.type` | `elastic-serverless-forwarder` |
| `agent.version` | Agent version (e.g., `v1.8.0`) |
| `agent.id` | Agent ARN (if possible) |