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.

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