[gRPC] Add sample client calls with dummy value to generated client file.

[daneshk]

Description:
Currently, when we are generating the code for below proto definition in client mode, the generated client looks like the below,

• helloworld.proto

 syntax = "proto3";

 package helloworld;

 service Greeter {
   rpc sayHello(HelloRequest) returns (HelloReply);
 }

 message HelloRequest {
   string name = 1;
 }

 message HelloReply {
   string message = 1;
 }

• Generated greeter_client.bal

GreeterClient ep = check new ("http://localhost:9090");

public function main() {
}

The proposed generated client code is add sample call with dummy data like the below,

import ballerina/io;
GreeterClient ep = check new ("http://localhost:9090");

public function main() returns error? {

    // Sample call to the Greeter/sayHello function.
    HelloRequest helloRequest = {name: "Sam"};
    HelloReply helloReply = check ep->sayHello(helloRequest);
    io:println(helloReply);
}