[Logs] Implement OtlpGrpcLogExporter and OtlpHttpLogExporter

[owent]

We need OTLP Log exporter of C++ in our company. But it seems has not plan for it right now.
Can I create a PR to implement OtlpGrpcLogExporter and OtlpHttpLogExporter to export opentelemetry.proto.collector.logs.v1.ExportLogsServiceRequest

https://github.com/open-telemetry/opentelemetry-proto/blob/main/opentelemetry/proto/collector/logs/v1/logs_service.proto

[lalitb]

@owent - yes it would be good to have this, please go ahead with PR. Thanks.

[ThomsonTan]

@owent thanks for the proposal. I have plan to work on OTLP exporter as well. Probably we could work on http and gRPC exporter separate to accelerate the process, or feel free to send PR with both exporters if you have them ready.

[owent]

@ThomsonTan OK, I can try to finish OtlpHttpLogExporter first.

[ThomsonTan]

@owent thanks. I will work on the OTLP gRPC log exporter which will still share many common code from your PR.