Adding service.version

[felixbarny]

Adding the service.version field as an user-configurable option.

Requested via elastic/ecs-logging-java#129

maybe you can just support a service.version to have a basic context of application name and version (easy to use and discover using the builder api or in config files)? imo, these two fileds are a 'must-have' when dealing wih logs or metrics

That makes sense to me. WDYT?

[apmmachine]

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• Start Time: 2022-05-09T04:36:00.378+0000

• Duration: 6 min 28 sec