Support custom_exe in fetch payloads

[bwatters-r7]

We support custom payloads in fetch payloads through the binary custom payload, but it only works with shellcode.
It would be nice to support a completely independent exe file for third-parties. Fetch payloads would be useful, because we could create a cmd/windows/http/multi/custom_exe payload, and it would just ask for an executable.
We should also remove use payload/cmd/windows/http/x64/download_exec because in theory, that is what we want, but it steps all over itself because it looks like it produces a binary for the download, so you have a command that downloads a binary to download another binary?