Incompatible return types for storage.get between local filesystem and GCS implementation

[vitorguidi]

As per GCP docs, the GET method returns a datetime object serialized as UTC string, for the last update time. Thus, the implementation for GCS in storage.get returns a string for ['updated'].

The local filesystem implementation, however, returns a raw datetime object. This generates inconsistency for two concrete implementations of the same contract.

The issue was noticed in #4341