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

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](https://github.com/google/clusterfuzz/blob/24f976051fbe6b02660cc02f9d5c87986b01c981/src/clusterfuzz/_internal/google_cloud_utils/storage.py#L381) in storage.get returns a string for ['updated'].

The local [filesystem implementation](https://github.com/google/clusterfuzz/blob/24f976051fbe6b02660cc02f9d5c87986b01c981/src/clusterfuzz/_internal/google_cloud_utils/storage.py#L646), however, returns a raw datetime object. This generates inconsistency for two concrete implementations of the same contract.

The issue was noticed in #4341