Add WifiOemConfigStoreMigrationHook class for OEM migration

Add a helper class (not part of the wifi mainline module) to migrate
config store data from existing OEM devices. This will be used when the
device upgrades from a build containing OEM impl of wifi to a
build containing wifi mainline module. OEM should implement this
class if they had any non-AOSP changes in the config store
format/field names on their devices.

Bug: 142394776
Test: Compiles
Change-Id: Ie11e0ffe8e4b8f1edcaa23d35284c3b564dc97f4