Add span links for message tracing

We are discussing adding [span link](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/overview.md#links-between-spans) support. One of the motivations for this is to be able to represent the batch processing of several messages. A transaction/span cannot have multiple parents, so span links are a mechanism to link that transaction to the many initiating messages.

This includes:
- [ ] Adding span links to messaging system instrumentation
- [ ] Revert the special case for SQS triggered AWS Lambda instrumentation that invocations with a single message are handled differently from invocations with batch messages.

## Spec issue
- [x] https://github.com/elastic/apm/issues/606

## Agent issues
- [x] https://github.com/elastic/apm-agent-java/issues/2489
- [x] https://github.com/elastic/apm-agent-dotnet/issues/1638
- [x] https://github.com/elastic/apm-agent-go/issues/1222
- [x] https://github.com/elastic/apm-agent-nodejs/issues/2593
- [x] https://github.com/elastic/apm-agent-python/issues/1473
- [ ] https://github.com/elastic/apm-agent-ruby/issues/1239
- [ ] https://github.com/elastic/apm-agent-php/issues/633