Adding dial options for PerRPCCredentials (#1225)

* Adding dial options for PerRPCCredentials

* Added tests for PerRPCCredentials

* Post-review updates

* post-review updates

rpc_util.go

@@ -46,6 +46,7 @@ import (
 
 	"golang.org/x/net/context"
 	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/credentials"
 	"google.golang.org/grpc/metadata"
 	"google.golang.org/grpc/peer"
 	"google.golang.org/grpc/stats"
@@ -141,6 +142,7 @@ type callInfo struct {
 	trailerMD metadata.MD
 	peer      *peer.Peer
 	traceInfo traceInfo // in trace.go
+	creds     credentials.PerRPCCredentials
 }
 
 var defaultCallInfo = callInfo{failFast: true}
@@ -207,6 +209,15 @@ func FailFast(failFast bool) CallOption {
 	})
 }
 
+// PerRPCCredentials returns a CallOption that sets credentials.PerRPCCredentials
+// for a call.
+func PerRPCCredentials(creds credentials.PerRPCCredentials) CallOption {
+	return beforeCall(func(c *callInfo) error {
+		c.creds = creds
+		return nil
+	})
+}
+
 // The format of the payload: compressed or not?
 type payloadFormat uint8