Implement Span Links API

[trentm]

As part of the larger span links work, a spec was added 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