zcash_client_backend: encoding: t-addr support - add a function to derive a t-addr from a seed and related utilities.

[gmale]

As a developer, I'd like to use the encoding module to decode/encode things related to t-addresses so that I can add basic t-address support to wallets so that users can receive (and shield) funds.

Requirements

At a minimum, we need to be able to create the default t-addr that wallets will use and check (i.e. account 0 and index 0). Ideally, we could also support wallets that already have keys derived and stored, meaning we could derive the address from the pk as well as the public key. We may also want to support multiple accounts or indexes.

Function description

inputs

seed: ByteArray - required.
account: number - optional. Typically defaults to zero.
coin_type: number - optional. Typically defaults to 133 for mainnet and 1 for testnet
addrss_index: number - preferred. Typically defaults to zero.

output

t-address: string

notes

We probably want related functions that work with the keys (public and private) instead of the seed. We probably also want utilities that derive and return these keys from a seed. That work can probably be split out into separate tickets.