Eliminate grpc dispatch proto marshaling overhead with Codec

[leonlnj]

Description

This MR seeks to improve the performance of grpc fiber by reducing proto marshalling overhead by using custom codec.

Fiber will no longer need to know the proto response of the target routes, any request sent to it will be expected to be in []byte. User of Fiber would need to marshal their own proto into Fiber.request.

The byte responses of route will simply be wrapped in the response and the user of Fiber would be expected to unmarshal them into the final proto message.

The default proto codec enforces proto.message on both the request and respond proto. Using the custom codec, this enforcement is loosen for type allowed in the codedc and the marshaling is skipped, making the below connection invocation possible with non proto message.

err := d.conn.Invoke(
		ctx,
		d.serviceMethod,
		grpcRequest.Payload(),
		response,
		grpc.Header(&responseHeader),
		grpc.CallContentSubtype(CodecName),
	)

Changes

• response.go and request.go - Payload() type changed from interface{} to byte[]
• grpc/dispatcher.go - Remove server reflection implementation
• grpc/codec.go - codec implementation

[pradithya]

LGTM. Thanks for this huge performance improvement!