Make cache lifetime for resources configurable #32
Description
Currently, the cache-control header is not configured consequently in all cases for Google Cloud Storage resources, which leads to published resourced getting a default expires header with 3600 seconds. That cache lifetime is too short and is disadvantageous in speed reports.
A plugin user should be able to configure a cache lifetime (for example 1 year) which is considered in all relevant situations where resources are imported, uploaded or updated, so that a proper cache-control header exists for each storage entry.
Furthermore it may be necessary to provide a way to update metadata for existing resources.