Fix #10 - ContainedBy Property Bug

[Tw1sm]

With the current main branch (version 0.4.0) when I upload a set of parsed results, I'm seeing these errors related to the ContainedBy attribute (#10). It does not appear to always be fatal, but in other cases I've seen BHCE/BHE fail to ingest 1+ JSON file due to it.

bloodhound-1  | {"level":"error","time":"2024-10-22T14:37:24.950684756Z","message":"Error decoding ein.User object: json: cannot unmarshal array into Go struct field User.ContainedBy of type ein.TypedPrincipal"}
bloodhound-1  | {"level":"error","time":"2024-10-22T14:37:24.950716298Z","message":"Error reading ingest file /opt/bloodhound/work/tmp/bh3374202504: json: cannot unmarshal array into Go struct field User.ContainedBy of type ein.TypedPrincipal"}
bloodhound-1  | {"level":"error","time":"2024-10-22T14:37:24.959774548Z","message":"Error decoding group object: json: cannot unmarshal array into Go struct field Group.ContainedBy of type ein.TypedPrincipal"}
bloodhound-1  | {"level":"error","time":"2024-10-22T14:37:24.959798381Z","message":"Error decoding group object: json: cannot unmarshal array into Go struct field Group.ContainedBy of type ein.TypedPrincipal"}
bloodhound-1  | {"level":"error","time":"2024-10-22T14:37:24.959806964Z","message":"Error decoding group object: json: cannot unmarshal array into Go struct field Group.ContainedBy of type ein.TypedPrincipal"}
bloodhound-1  | {"level":"error","time":"2024-10-22T14:37:24.959813423Z","message":"Error decoding group object: json: cannot unmarshal array into Go struct field Group.ContainedBy of type ein.TypedPrincipal"}

With this PR branch and the same log data, none of those errors now appear.