cmd/frontend-gcp: add GCP wrapper for frontend binary
cmd/frontend-gcp: add GCP wrapper for frontend binary

The only distinction between this and cmd/frontend is that frontend-gcp
can store its Let's Encrypt certificates in a Cloud Storage bucket.

Change-Id: I05a7885fa6cd7a9997fd1dfa6b8ab53b73603d12
Reviewed-on: https://upspin-review.googlesource.com/9241
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/gcp,upspin/gcp