Let devs install microsoft/go similarly to `go install golang.org/dl/go1.16.6`

See https://golang.org/doc/manage-install#installing-multiple. (A partner team pointed it out to us as a way they get Go. In particular, to compare how different Go versions go when something strange is going on.)

https://github.com/golang/dl is a mirror of the golang.org/dl module's repo. It has a package for each version of Go: a small wrapper that can download that specific version to e.g. ~/sdk/go1.16.6, and after that, it redirects go1.16.6 [...] commands to that Go binary it downloaded.

The simplest-seeming thing to do would be to copy dl and provide a https://github.com/microsoft/go-dl module with e.g. a microsoft-go1.16.6 package that points at our blob storage instead of the tar.gz/zip files available on dl.google.com/go/.

Start by reading the linked Go documentation and the golang/dl repository, especially its per-version package approach. Determine how a Microsoft Go module would download from Microsoft blob storage and redirect commands to that version; done means developers can install and invoke a selected Microsoft Go version similarly to golang.org/dl.