Support C2PA 2.2 BMFF Update Manifest

C2PA 2.2 specifies split manifests for update manifests.  This is to add

- Add required store functions to append, and split manifests stores
- Update BMFF asset handler reading to returned merged stores
- Update BMFF assert handling to write update manifest in separate manifest store.
- Merge and/or delete BMFF manifest stores when writing ordinary manifests