Introduce `service.namespace`

Changes to implement:
1. Add [service.namespace](https://opentelemetry.io/docs/specs/otel/resource/semantic_conventions/#service-experimental) field to apm-data model & inputs
2. Add `service.namespace` to the service aggregation key in `apm-aggregation`

The `service.namespace` field will be subject to change until it is stabilised in OTel semantic conventions, and hence the field will be in Technical Preview.