Support Zcash Unified Addresses (UAs)

[Zcash Unified Addresses](https://electriccoin.co/blog/unified-addresses-in-zcash-explained) have been around for several months, but keybase won't let us post them to our profile yet. Please add this support.

This is similar in spirit to #14364, where shielded addresses were upgraded from Sprout to Sapling. But now with the new Orchard pool, which has its own addresses, Zcash has also added Unified Addresses (UAs), which should in theory be the last kind of address, even if new pools are created in the future. But best of all, because UAs encode multiple pools into a single address, it provides for folks to send payments to each other, optimizing for the shielded pool that the sender already has funds in. This prevents funds having to cross shielded pool boundaries and thus makes it more likely that the entire transaction is totally shielded.

When keybase supports this, zcash users will enjoy greater privacy.

No file, test, or entry point is identified. Start by reviewing the linked Unified Addresses explanation and issue #14364, then locate the existing Zcash shielded-address profile support. Done means Zcash Unified Addresses can be posted to a Keybase profile.

Written by the indexing model from the issue text.