Proposal: new getter: `get_filesystems`

[decoupca]

Based on this discussion.

I propose a new getter called get_filesystems.

Example data structure for IOS:

{
    'flash:': {
        'capacity': 1621966848,
        'space_used': 878018560,
        'space_remaining': 743948288,
        'is_primary': True, # derived from `show file systems`, denoted with asterisk
        'mount_point': 'flash:', # Redundant for IOS but useful for junos or others
        'flags': 'rw',
        'file_list': [
            {
                'file_name': 'cat3k_caa-universalk9.16.12.05b.SPA.bin',
                'size': 480018036,
                'mtime': '2021-08-12T23:53:40Z', # Should be a datetime object
                'mode': '-rw-',
            },
            {
                ...
            }
        ],
    },
    'crashinfo:': {
        'capacity': 248354816,
        'space_used': 19483648,
        'space_free': 228871168,
        'is_primary': False,
        'mount_point': 'crashinfo:',
        'flags': 'rw',
        'file_list': [
            {
                'file_name': 'last_systemreport_log',
                'size': 357,
                'mtime': '2021-08-12T23:53:40Z',
                'mode': '-rwx',
            },
            {
                ...
            }
        ]
    }
}

I don't have enough experience with Junos, Arista, etc. to know if/how they could support this data structure. IMO the most important values are the filesystem name, its capacity, space used, and its flags (rw/ro/etc). The file listing would be nice but I would give that up if it's not feasible. Also some way to determine which filesystem is "primary", i.e., which is used for main storage of images, etc.