Add support for subdirectories in request path

Seems GrpCurl is missing support for being able to add a sub directory for the request call.

Tried
- Adding to address param (obv throws an error)
- Adding as fully qualified symbol is not supported
- Adding as a customer header for :path does not work

Maybe i am not doing it correctly but nothing here seems to work

option 1
```text
./grpcurl -plaintext -vv -v -proto ./linuxcode/api.proto -H "Authorization: Bearer ey..." -d '{"RegionName":"Dev","ResourceName":"Test"}' "internal.company.net:5050/platform/" "Settings.GetSettings"
```

option 2
```text
./grpcurl -plaintext -vv -v -proto ./linuxcode/api.proto -H "Authorization: Bearer ey..." -d '{"RegionName":"Dev","ResourceName":"Test"}' "internal.company.net:5050" "platform/Settings.GetSettings"
```

option 3
```text
./grpcurl -plaintext -vv -v -proto ./linuxcode/api.proto -rpc-header ':path: platform' -H "Authorization: Bearer ey..." -d '{"RegionName":"Dev","ResourceName":"Test"}' "internal.company.net:5050" "Settings.GetSettings"
```

option 1
```text
./grpcurl -plaintext -vv -v -proto ./linuxcode/api.proto -H ':path: platform' -H "Authorization: Bearer ey..." -d '{"RegionName":"Dev","ResourceName":"Test"}' "internal.company.net:5050" "Settings.GetSettings"
```

Any help or in site to this being a bug would be great.

Background
in our .net core application to get it all working correctly we inject a custom httpHandler that appends the subdirectory to the outgoing request to the GRPC service. This all works and is actually presented as the way by Microsoft: https://docs.microsoft.com/en-us/aspnet/core/grpc/troubleshoot?view=aspnetcore-6.0#calling-grpc-services-hosted-in-a-sub-directory