gnmic supports exporting subscription updates to multiple local files

A file output can be defined using the below format in gnmic config file under outputs section:

    type: file # required
    filename: /path/to/filename
    file-type: stdout # or stderr

The file output can be used to write to file on the disk, to stdout or to stderr.

For a disk file, a file name is required.

For stdout or stderr, only file-type is required.