Add new proof of access put logic

Add new APIs to ManagedStorage which allow a caller to "upload" a file by proving they have access to it, without needed to upload the whole file. They ask to put a file (supplying the checksums for the entire file) and are prompted to respond with checksums from a random byte range in the file. If the checksums match, then the put succeed as if the whole file were uploaded.
Marginally :)
The problem I have is that the name says nothing to me about the fact that it's referring to an existing blob. I'm not going to get hung up over it.