Add native OpenTelemetry Protocol (OTLP) intake support to APM Server.

We will piggyback on the work done for #3984 to serve on the same port as used for receiving data from Elastic APM agents. Initially we will support OTLP/gRPC, later we can consider extending this to OTLP/HTTP.

The APM Server must be capable of receiving at least traces and metrics over OTLP, as we do with the elastic opentelemetry-collector exporter and we should also consider extending this to receiving logs.