[Feature]: set StoreLoader in v2 app

[randygrok]

Summary

V2 did not have the StoreLoader functions that allowed the application to upgrade the state on governance software upgrade proposals.

Problem Definition

We don't currently have the SetStoreLoader or equivalent on v2, so we could not make upgrade proposals.

Check simapp/v2/upgrades.go for reference.

Also, use the v1 app to see how BaseApp used it.

Proposed Feature

Bring StoreLoader into v2.