Go Module Proxy

[alecthomas]

We use Artifactory for all other languages, but for Go we'll need to implement the Go Module Proxy protocol.

[js-murph]

Implemented in https://github.com/block/cachew/pull/43/files

[js-murph]

Re-implemented in #55 to use the embedded goproxy module.

Further work on-going to explore private-repo cloning integrated with the git strategy.