Add stat_json() to get the raw file info from Artifactory

[allburov]

Closes #488, co-authored by @donhui.

#488 wanted path and downloadUri from the File Info API. Rather than add them to the ArtifactoryFileStat namedtuple, this exposes the raw response: stat() keeps a fixed documented field set, like pathlib.Path.stat() returning an os.stat_result, and stat_json() gives everything the server sends — including any field JFrog adds later.

The accessor already had get_stat_json(); it was just unreachable without touching path._accessor.

stat = path.stat_json()
print(stat["downloadUri"])
print(stat["path"])