Make public FileSystem registry

https://github.com/apache/beam/blob/5743a37/sdks/go/pkg/beam/io/textio/filesystem.go

The current beam/io/textio package includes a useful FileSystem interface and corresponding RegisterFileSystem function.  The textio package uses this internally to expose a Read(beam.Scope, string) function that will work for any file path corresponding to a registered FileSystem.

It would be extremely useful to expose the FileSystem interface outside of just the textio package and add global analogs for each of the FileSystem interface functions using the registry.  This would allow for easier implementation of other file reading sources.