[download] allow get_file to only evaluate part of the hash

Also changes the default auto algorithm to sha256 if the hash is partial
4 jobs for get-file-partial-hash in 4 minutes and 16 seconds (queued for 5 seconds)
Status Name Job ID Coverage
  Build
passed build_linux_37 #225814
docker

00:02:14

75.0%
passed build_linux_38 #225815
docker

00:02:14

75.0%
passed build_macos_intel_37 #225812
macos intel

00:04:14

76.0%
passed build_macos_intel_38 #225813
macos intel

00:04:13

76.0%