Implement Span Links API

As part of the larger span links work, [a spec was added](https://github.com/elastic/apm/pull/620) for optionally adding an API to agents for adding span links at span/transaction creation time. This includes:

- [ ] Optionally provide an API to specify span links at Span/Transaction creation time. (Node.js will, Java will not, etc.)
- [ ] Support span links being specified during OTel span creation, for those agents that have implemented the OTel Bridge (Python, Java, .NET)

# Spec issue

- [x] https://github.com/elastic/apm/issues/594

## Agent Issues
- [x] https://github.com/elastic/apm-agent-java/issues/2613
- [x] https://github.com/elastic/apm-agent-dotnet/issues/1691
- [x] https://github.com/elastic/apm-agent-nodejs/issues/2673
- [x] https://github.com/elastic/apm-agent-python/issues/1541
- [x] https://github.com/elastic/apm-agent-go/issues/1243
- [ ] https://github.com/elastic/apm-agent-php/issues/677
- [x] https://github.com/elastic/apm-agent-ruby/issues/1256