Enable wallet initialization from a UFVK

[str4d]

This issue is about creating a new Zashi wallet that just contains a UFVK, and no seed (but with assumed access to the seed). I'm not entirely certain whether #205 means we already have support for this; at a minimum we will still need changes to handle the lack of a seed around database migrations.

Now that we have address book encryption, when initializing the SDK with a UFVK we'd also want to enable bringing in the corresponding address book key (whether that is separately, or via a UFVK metadata item). We might also want or need a seed fingerprint.

[str4d]

Closed by #1517.