aconfig: add create storage command

Add a new aconfig command called create-storage which takes a number
of aconfig cache files that belong to a specific container and produces
storage files.

Add a new module called storage (src/storage/mod.rs) as the entry point
of storage files generation. FlagPackage struct is defined as an
intermediate data structure that will be used to drive all storage files creation.

Add a unit test to lock down FlagPackage creation behaviors.

Bug: b/312243587
Test: atest aconfig.test

Change-Id: Ia7e9f68237ea903f295ac7891c923f6a39f3422d