Set update time in LocalStorageHelper/FakeStorageRpc

[hosainnet]

Is your feature request related to a problem? Please describe.

When trying to test any time-based/process once filters (such as https://github.com/spring-cloud/spring-cloud-gcp/blob/master/spring-cloud-gcp-storage/src/main/java/org/springframework/cloud/gcp/storage/integration/filters/GcsPersistentAcceptOnceFileListFilter.java#L31) via FakeStorageRpc it doesn't work as expected (NPE/fixed time is set)

*Describe the solution you'd like

Any time we write a blob, we need update time to be set on the storage object

Describe alternatives you've considered

Not using those filters in tests, but that means our test is not realstic due to different behaviours in test/main code

Additional context
None

[hosainnet]

PR: #174

[hosainnet]

@frankyn @JesseLovelace any update on this? I already I have a PR #174
Many thanks :)