lib/proto: a Starlark package for protobuf processing (#318)

This change defines an optional Starlark module for encoding
and decoding protocol buffers (https://developers.google.com/protocol-buffers/).

It adds a dependency on google.golang.org/protobuf.

No promises of API stability yet: there are a number of TODOs
to be resolved and questions to be answered first, but I would
like to get this into the hands of the community. See discussion in
https://github.com/stripe/skycfg/issues/23.

Fixes #309
diff --git a/go.mod b/go.mod
index 4302c6b..50bc000 100644
--- a/go.mod
+++ b/go.mod
@@ -7,4 +7,5 @@
 	github.com/chzyer/readline v0.0.0-20180603132655-2972be24d48e
 	github.com/chzyer/test v0.0.0-20180213035817-a1ea475d72b1 // indirect
 	golang.org/x/sys v0.0.0-20200803210538-64077c9b5642
+	google.golang.org/protobuf v1.25.0
 )