[FAB-7567] Golang client reference implementation

This change set adds a reference implementation of a service discovery
client in Golang.

This is to be used by an SDK, or a CLI. 

The change set is quite large but this is mostly due to test code

Change-Id: I1b9685140c690f2af37e6332e281de4fb63b064b
Signed-off-by: yacovm <yacovm@il.ibm.com>

Author: yacovm

discovery/client/api.go

@@ -0,0 +1,67 @@
+/*
+Copyright IBM Corp. All Rights Reserved.
+
+SPDX-License-Identifier: Apache-2.0
+*/
+
+package discovery
+
+import (
+	"github.com/hyperledger/fabric/protos/discovery"
+	"github.com/hyperledger/fabric/protos/gossip"
+	"github.com/pkg/errors"
+	"golang.org/x/net/context"
+	"google.golang.org/grpc"
+)
+
+var (
+	// ErrNotFound defines an error that means that an element wasn't found
+	ErrNotFound = errors.New("not found")
+)
+
+// Signer signs a message and returns the signature and nil,
+// or nil and error on failure
+type Signer func(msg []byte) ([]byte, error)
+
+// Dialer connects to the server
+type Dialer func() (*grpc.ClientConn, error)
+
+// Client defines the client-side API of the discovery service
+type Client interface {
+	// Send sends the Request and returns the response, or error on failure
+	Send(context.Context, *Request) (Response, error)
+}
+
+// Response aggregates several responses from the discovery service
+type Response interface {
+	// ForChannel returns a ChannelResponse in the context of a given channel
+	ForChannel(string) ChannelResponse
+}
+
+// ChannelResponse aggregates responses for a given channel
+type ChannelResponse interface {
+	// Config returns a response for a config query, or error if something went wrong
+	Config() (*discovery.ConfigResult, error)
+
+	// Peers returns a response for a peer membership query, or error if something went wrong
+	Peers() ([]*Peer, error)
+
+	// Endorsers returns the response for an endorser query for a given
+	// chaincode in a given channel context, or error if something went wrong.
+	// The method returns a random set of endorsers, such that signatures from all of them
+	// combined, satisfy the endorsement policy.
+	Endorsers(string) (Endorsers, error)
+}
+
+// Endorsers defines a set of peers that are sufficient
+// for satisfying some chaincode's endorsement policy
+type Endorsers []*Peer
+
+// Peer aggregates identity, membership and channel-scoped information
+// of a certain peer.
+type Peer struct {
+	MSPID            string
+	AliveMessage     *gossip.SignedGossipMessage
+	StateInfoMessage *gossip.SignedGossipMessage
+	Identity         []byte
+}