Support gRPC transport in OTLP

**Describe the current state**

We only support HTTP as transport for voth JSON and protobuf messages.
These are mandatory as per the specification.

gRPC transport is optional but its widely used and we should be able to support it.

**Describe the desired state**

Add gRPC transport support in `src/otlp.zig`.
There is a library that might be usedfor this: https://github.com/ziglana/gRPC-zig