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/plugins/b3PluginContext.h

@@ -18,6 +18,9 @@ struct b3PluginContext
 	int m_numMouseEvents;
 	const struct b3Notification* m_notifications;
 	int m_numNotifications;
+	
+	//only used for grpc/processClientCommands
+	class PhysicsCommandProcessorInterface* m_rpcCommandProcessorInterface;
 };