Add HTTP API to retrieve all file statuses for a given folder

[meejah]

GridSync needs, at minimum, the size of every file in a folder in order to be able to ask ZKAPAuthorizer how many storage-time credits will be used to store those files.

Add an endpoint returning the status of every file in a folder. Currently, that status only needs to include the size.

Something like GET /v1/magic-folder/<folder-name>/file-status or similar. Returning data like:

{
    "rel/path": {
        "size": 1234
    }
}