Adds a stream interceptor to keep state between the send and receive calls

[nadinelyab]

Adds an new option WithStreamInterceptorProviderFunc where a StreamInterceptorProviderFunc can be specified. With this option a new Stream Interceptor will be created for each new stream created. The Stream Interceptor's Send and Recv functions will be called when a message is sent or received. Unlike the StreamRecvMsgInterceptFunc and StreamMessageProviderFunc, the StreamInterceptor can keep state that both Send and Recv functions may need access to.

Here is an example of how to use the Stream Interceptor:
`

    type ABCStreamInterceptor struct {
	MsgRcvCount  int
	MsgSendCount int
            MsgChan 	 chan int
}

func (s *ABCStreamInterceptor) Send(callData *CallData) (*dynamic.Message, error) {
	defer func() { s.MsgSendCount++ }()
	switch s.MsgSendCount {
	case 0:
		return dynamic.AsDynamicMessage(&exampleapi.Request{
			Num: 1,
		})
	case 1:
		num := <-s.MsgChan
		return dynamic.AsDynamicMessage(&exampleapi.Request{
			Num: num,
		})
	default:
		return nil, fmt.Errorf("unknown msg send")
	}
}

func (s *ABCStreamInterceptor) Recv(msg *dynamic.Message, err error) error {
	if msg == nil {
		return err
	}
	s.MsgRcvCount++

	reply := exampleapi.Response{}
	err = msg.ConvertTo(reply)
	if err != nil {
		return err
	}

	switch s.MsgRcvCount {
	case 0:
		s.MsgChan <- reply.Num
		return nil
	case 1:
		return runner.ErrEndStream
	default:
		return fmt.Errorf("unknown msg recv")
	}
}

WithStreamInterceptorProviderFunc(func NewABCInterceptor() StreamInterceptor {
	return &ABCStreamInterceptor{
			MsgChan:          make(chan exampleapi.response, 1),
		}
	}
})`

[nadinelyab]

@bojand let me know if you have some time to review. Thank you so much!

[bojand]

Hello, thanks for the PR. I'll try and take a look at this soon.