allow pybullet to connect to GRPC server. (need to use flag --enable_grpc in premake build system)

add grpcPlugin, it can work in GUI, SHARED_MEMORY_SERVER, DIRECT and other modes.
example script to start server from pybullet:
import pybullet as p
p.connect(p.GUI)
#if statically linked plugin
id = p.loadPlugin("grpcPlugin")
#dynamics loading the plugin
#id = p.loadPlugin("E:/develop/bullet3/bin/pybullet_grpcPlugin_vs2010_x64_debug.dll", postFix="_grpcPlugin")

#start the GRPC server at hostname, port
if (id>=0):
	p.executePluginCommand(id, "localhost:1234")

Only in DIRECT mode, since there is no 'ping' you need to call to handle RCPs:
numRPC = 10
while (1):
	p.executePluginCommand(id, intArgs=[numRPC])

examples/SharedMemory/PhysicsServerSharedMemory.cpp

@@ -249,6 +249,8 @@ void PhysicsServerSharedMemory::reportNotifications()
 
 void PhysicsServerSharedMemory::processClientCommands()
 {
+	m_data->m_commandProcessor->processClientCommands();
+
     for (int block = 0;block<MAX_SHARED_MEMORY_BLOCKS;block++)
     {