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

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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)

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