Implement `GET /v1/<folder-identifier>/snapshots`

[exarkun]

magic-folder should expose snapshot DAGs for all files in a magic folder

This should look something like

GET /v1/<folder-identifier>/snapshots
Accept: application/json

200 OK
Content-Type: application/json

{ “filename1”: [
    {... snapshot object … }
  , {... snapshot object … }
  , …
  ],
  “filename2”: ...
]