Move the cache for `CRM_Extension_Browser` out of the filesystem and use a `SqlGroup` instead

The current code in CRM_Extension_Browser is coded specifically for an adhoc, file-based caching logic. The recommendation standard is basically to change the backing/storage from a JSON file to the civicrm_cachetable (at least, for the typical usage).

Ref #2

Edited Jun 11, 2022 by bgm

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