Auto-infer destination.service.resource and adapt public API

https://github.com/elastic/apm/pull/436 defines an update on the SPEC for destination fields.
APM agents need to comply with these changes, which includes the following:

- Infer destination.service.resource based on other fields
- Add a public API to set destination.service.resource
- As destination.service.name and destination.service.type are not used anywhere in the product, I propose to deprecate and remove them also in the public API (if already added there)
- Add a public API to start an exit span. This auto-infers the destination.service.resource based on the subtype if not set otherwise. Avoids creating nested exit spans and makes the service map work with those custom spans by default, without having to set any destination fields.

## Spec issue
- [x] https://github.com/elastic/apm/pull/436

## Agent issues
- [x] https://github.com/elastic/apm-agent-java/issues/1848
- [x] https://github.com/elastic/apm-agent-dotnet/issues/1330
- [x] https://github.com/elastic/apm-agent-go/issues/976
- [ ] https://github.com/elastic/apm-agent-php/issues/426
- [x] https://github.com/elastic/apm-agent-ruby/issues/1022
- [ ] https://github.com/elastic/apm-agent-nodejs/issues/2103
- [x] https://github.com/elastic/apm-agent-python/issues/1159