Extend Intake to accept FaaS / Serverless context fields

[AlexanderWert]

With adding AWS Lambda support to our agents we need additional context in the intake API that accepts function as a service (faas)-specific fields.

Fields to be added:

Field	Example	Metadata	Transaction	Description
service.id	e.g. arn:aws:lambda:us-west-2:123456789012:function:my-function	yes	yes	Add service.id(available in ECS) to the intake model. Value for Lambda: The ARN of the function without alias suffix.
faas.trigger.type	http	no	yes	The trigger type.
faas.execution	af9aa4-a6...	no	yes	The AWS request ID of the function invocation
faas.coldstart	true / false	no	yes	Boolean value indicating whether a Lambda function invocation was a cold start or not.
faas.trigger.reuqest_id	af9aa4-a6...	no	yes	The ID of the origin trigger request
cloud.origin.*	-	no	yes	Allow to self-nest all cloud fields under cloud.origin to specify trigger cloud data. See elastic/ecs#1542
service.origin.*	-	no	yes	Allow to self-nest all service fields under service.origin to specify trigger service data. See elastic/ecs#1542

Names and fields are inspired by the OpenTelemetry specification for functions as a service.

There is a PR for starting the RFC process to add these fields also to ECS.

[axw]

Also relevant: #5456