Standalone file adapter lacks Windows UNC path support

[chrisd8088]

Describe the bug
As reported in discussion #5692, the git-lfs-standalone-file adapter parses input file:// URLs, and then only uses the url.Path element, so file:// URLs corresponding to Windows UNC file paths, where the hostname is parsed into the url.Host element, are unsupported.

To Reproduce
Steps to reproduce the behaviour:

1. On Windows, attempt a Git clone using a file://<host>/<path> URL corresponding to a Windows UNC path to another host, e.g.:

   $ git clone "file://DESKTOP-D3J67S2/Temas_Git/Repositorio"
   ...
   Error downloading object: Doc_01.txt (e470fa0): Smudge error: Error downloading Doc_01.txt (e470fa0d001e50b3ec3088022462a94ea7c87dd80106411b7d120f90b379e977): error initializing custom adapter "lfs-standalone-file" worker 0: [0] error creating handler: chdir /Temas_Git/Repositorio: El sistema no puede encontrar la ruta especificada.
   

Expected behaviour
The standalone transfer adapter should convert the file://host/path URL into a Windows UNC path and allow the operation to succeed. For instance, file://DESKTOP-D3J67S2/Temas_Git/Repositorio would become \\DESKTOP-D3J67S2\Temas_Git\Repositorio.

System environment
Windows

Output of git lfs env

Endpoint=file://DESKTOP-D3J67S2/Temas_Git/Repositorio (auth=none)
LocalWorkingDir=C:\Users\Japo\Desktop\Clon\Repositorio
LocalGitDir=C:\Users\Japo\Desktop\Clon\Repositorio.git
LocalGitStorageDir=C:\Users\Japo\Desktop\Clon\Repositorio.git
LocalMediaDir=C:\Users\Japo\Desktop\Clon\Repositorio.git\lfs\objects
LocalReferenceDirs=
TempDir=C:\Users\Japo\Desktop\Clon\Repositorio.git\lfs\tmp
ConcurrentTransfers=8
TusTransfers=false
BasicTransfersOnly=false
SkipDownloadErrors=false
FetchRecentAlways=false
FetchRecentRefsDays=7
FetchRecentCommitsDays=0
FetchRecentRefsIncludeRemotes=true
PruneOffsetDays=3
PruneVerifyRemoteAlways=false
PruneRemoteName=origin
LfsStorageDir=C:\Users\Japo\Desktop\Clon\Repositorio.git\lfs
AccessDownload=none
AccessUpload=none
DownloadTransfers=basic,lfs-standalone-file,ssh
UploadTransfers=basic,lfs-standalone-file,ssh
GIT_EXEC_PATH=C:/Program Files/Git/mingw64/libexec/git-core
GIT_TRACE=1
git config filter.lfs.process = "git-lfs filter-process"
git config filter.lfs.smudge = "git-lfs smudge -- %f"
git config filter.lfs.clean = "git-lfs clean -- %f"

Additional context
Conversion of a Windows file:// URL into a UNC path is described in this blog post, as mentioned in golang/go#32456.

To do this, we could borrow Go's implementation from their command-line tools.

[vruss]

file:////<host>/<path> seems to work as a workaround for me