Support centrally defined service labels

**Is your feature request related to a problem? Please describe.**

Users sometimes have a need to specify additional labels for instrumented services, e.g. the organisation/department/team which owns the service.

At the moment it is only possible to specify global labels locally to each agent, i.e. by setting the `ELASTIC_APM_GLOBAL_LABELS` environment variable or language-specific equivalents (system properties, etc.)

**Describe the solution you'd like**

We should be able to define a mapping of `service.name` to global labels through agent central config. e.g. https://discuss.elastic.co/t/setup-global-labels-in-remote-apm-agent-configuration/328112

**Describe alternatives you've considered**

Users could attach additional labels by customising the APM ingest pipelines.