Dynamic gRPC client support

[nv-hwoo]

This is an initial effort to support streaming NIMs using gRPC protocol. The PR introduces a new gRPC client and client backend that enables PA to send requests to gRPC server dynamically without requiring PA to pre-compile the protobuf to generate codes for gRPC client stub.

The current version of the client only supports bidirectional streaming RPC and will support other RPC types in the future.

Brief Example

1. User writes a short script (with language of their choice) that creates protobuf messages to send and writes it to a file stream:

# input_stream.py
import  sys

def generate_msgs():
    # generate protobuf messages
    yield ...
    yield ...
    ...

for msg in generate_msgs():
    serialized = msg.SerializeToString()
    # encode msg size
    sys.stdout.buffer.write(len(serialized).to_bytes(4, byteorder=sys.byteorder))
    # encode actual msg
    sys.stdout.buffer.write(serialized)
    sys.stdout.buffer.flush()

2. Create JSON file to specify the command to run the script (e.g. inputs.json):

{
  "data": [
    {
      "ipc_stream": "python3 input_stream.py"
    }
  ]
}

4. Run Perf Analyzer:

perf_analyzer -u localhost:8004 --service-kind dynamic_grpc --streaming --input-data inputs.json --proto <path/to/proto> --rpc <full grpc method name>

PA will execute the user provided command, read/store the messages generated by user script, and send them over to server (in the same order as the user sent them).