Allow users to mock functions returning storage::*Metadata values.

[coryan]

For "good reasons" the storage::*Metadata classes do not have constructors or accessors that can set every field. Many of these classes have fields that are read-only, that is, setting the fields in a Update or Patch request is always an error.

The only way to initialize these values is through the functions to convert a string in the JSON object format into storage::*Metadata object. But these functions live in the internal:: namespace, and applications should not be calling functions in the internal:: namespace, not even for mocks.

Maybe we can create functions in a storage_mocks namespace, which at least would make it obvious that they are (a) not for general use, and (b) part of our public APIs that we intend to support.

[devjgm]

To be fixed as part of #7142