x/build/cmd/buildlet: add the ability to download an unzipped file from a URL

[cagedmantis]

As part of #47521, we are shifting to a model where we try to avoid streaming files through the coordinator on to the buildlet. The buildlet has a /write endpoint which can be used to write a single file to a machine. It does not have the functionality to pass a URL where the buildlet can download a file and store it on the file system of the host. We should add the ability to pass a URL to the buildlet so that the caller would not have to stream the file to the buildlet.

@golang/release

[dmitshur]

This may also help with #51057 as I understand.