services/fs: Handle the drive letter in connection string for Windows

[Prnyself]

For now, we use : as the separator for the connection string.

But in Windows os, the path is usually like: C:\\folder\to\file.txt, which cannot be parsed as expected for now.

Moreover, we should specify the format of Windows-fs-like connection string.

ref: https://github.com/beyondstorage/go-storage/blob/master/services/new.go#L157

[Xuanwo]

GSP-90 requires a quote for values that contains : or &. Is fs://"C:\\folder\to\file.txt" works correctly?