[#286] client: Add TLS options

Add `WithTLSConfig` option to client.
If it is not nil then client will
try to open secured connection.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

rpc/client/options.go

@@ -1,6 +1,7 @@
 package client
 
 import (
+	"crypto/tls"
 	"time"
 
 	"google.golang.org/grpc"
@@ -14,6 +15,8 @@ type cfg struct {
 
 	dialTimeout time.Duration
 
+	tlsCfg *tls.Config
+
 	conn *grpc.ClientConn
 }
 
@@ -49,6 +52,18 @@ func WithDialTimeout(v time.Duration) Option {
 	}
 }
 
+// WithTLSCfg returns option to specify
+// TLS configuration.
+//
+// Ignored if WithGRPCConn is provided.
+func WithTLSCfg(v *tls.Config) Option {
+	return func(c *cfg) {
+		if v != nil {
+			c.tlsCfg = v
+		}
+	}
+}
+
 // WithGRPCConn returns option to specify
 // gRPC virtual connection.
 func WithGRPCConn(v *grpc.ClientConn) Option {