model: align stacktrace with ECS

[axw]

ECS does not define structured stack traces yet, only defines a text stacktrace as part of the ECS error 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 elastic/ecs#154