support JsonPrintOptions on RPCContext (currently protobuf only)

[holmes1412]

Support setting JsonPrintOptions through RPCContext. Currently only implement protobuf and will support thrift in the future.

Proto:

syntax="proto3";                                                                
                                                                                
message EchoRequest {                                                           
    string message = 1;                                                         
    string name = 2;                                                            
};                                                                              

message EchoResponse {                                                          
    string message = 1;                                                         
    int32 state = 2;                                                            
    int32 error = 3;                                                            
    repeated Log log = 4;                                                       
};

Usage:

class ExampleServiceImpl : public Example::Service                              
{                                                                               
public:                                                                         
    void Echo(EchoRequest *req, EchoResponse *resp, RPCContext *ctx) override   
    {                                                                           
        resp->set_message("Hi back");                                           
        resp->set_state(1412);
        resp->set_error(0);                                                  
        ctx->set_json_always_print_primitive_fields(true);                      
        ctx->set_json_add_whitespace(true); 
        ...

Origin Output:

{"message":"Hi back","state":1412}

After setting json options on RPCContext:

{
 "message": "Hi back",
 "state": 1412,
 "error": 0,
 "log": []
}