model: align stacktrace with ECS

ECS does not define structured stack traces yet, only defines a text stacktrace as part of the [ECS error](https://www.elastic.co/guide/en/ecs/current/ecs-error.html) event.

APM defines stacktraces on `error.exception`, `error.log`, `profile.stack` and `span.stacktrace`. The stacktraces are not indexed, therefore changes might not be considered breaking changes. Profile stack traces may not be aligned with those on errors/spans; we should ensure they are.

We should define an ECS stacktrace concept that does not conflict with current ECS `error.stacktrace` (keyword), e.g. define a reusable stackframes field set that can be nested under other namespaces. We should investigate if we can use the current APM stacktrace structure and bring it to ECS as-is. If this is not possible see if changes can be made in a way to not break the APM UI (most values are simply shown).

See also https://github.com/elastic/ecs/issues/154