Add an OTLP (gRPC) logger to Envoy

Adding the OTLP logger will allow integration with more 3rd party monitoring frameworks, especially when using OT collector to convert them to whichever format needed.

Although OpenTelemetry logging isn't going to be part of the GA and the C++ SDK isn't ready yet, we don't actually need them in order to export OTLP logs - as long as the [OTLP logs proto](https://github.com/open-telemetry/opentelemetry-proto/blob/master/opentelemetry/proto/common/v1/common.proto) is stable.
The logs format is actually already available in the Amazon managed OT collector as an exporter, and Microsoft are also working on something similar, so it shouldn't change much - either way we'll get the OT community guarantee for backwards compatibility.

The general idea is to take the existing gRPC logger and just output a different format.

FYI @htuch @yanavlasov