[ZIP 321] Update spec to cover Unified Addresses

[daira]

Original description:

The idea is that a ZIP 321 URI can include multiple address types, and can therefore act as a substitute for an address that can more smoothly accommodate address upgrades.

Now that we have unified addresses which achieve the same effect as the above, we should make sure that they are explicitly allowed by ZIP 321.

Adding example URIs that use Unified Addresses is #753.

[daira]

The current specification says:

Implementations SHOULD check that each instance of zcashaddress is a valid string encoding of either:

• a Zcash transparent address, using Base58Check [7] as defined in [12]; or
• a Zcash Sapling address, using Bech32 [8] as defined in [13].

New address formats may be added in future. If the context of whether the payment URI is intended for Testnet or Mainnet is available, then each address SHOULD be checked to be for the correct network.

There isn't really any reason to restrict what new address types can be used, so we should also generalize the wording so that anything described as an address in section 5.6 of the protocol spec is allowed, unless (as for Sprout) it is explicitly prohibited. This should reduce the need to update the ZIP in future.