syncfeatures: add support for TLS

Until now syncfeatures could only connect to insecure endpoints. This CL
allows it to connect to gRPC endpoints with server-side TLS
authentication.

Bug: twpowertools:62
Change-Id: I1d0fd6bcf2aead47c152f66a10be538f6655ca7c
diff --git a/go.mod b/go.mod
index 35d81e7..b1b3e67 100644
--- a/go.mod
+++ b/go.mod
@@ -6,6 +6,7 @@
 	github.com/Masterminds/semver/v3 v3.1.1
 	github.com/ReneKroon/ttlcache/v2 v2.8.0
 	github.com/go-sql-driver/mysql v1.6.0
+	github.com/johnsiilver/getcert v0.0.0-20201005022436-57fe9706afac // indirect
 	google.golang.org/api v0.54.0
 	google.golang.org/grpc v1.40.0
 	google.golang.org/protobuf v1.27.1