Introduce `service.namespace`

[simitt]

Changes to implement:

1. Add service.namespace 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.

[simitt]

As discussed today with @felixbarny , this needs more discussion and design, and therefore removed from 8.10 milestone.

[brettmc]

Related issue: https://github.com/elastic/apm/issues/544