zinniad: validate the wallet address

[bajtos]

At startup, parse the wallet address provided by the user and check that it's a valid mainnet address.

Resources:

• https://crates.io/crates/fvm_actor_utils
• https://docs.rs/fvm_shared/3.1.0/fvm_shared/address/enum.Network.html#method.parse_address
• https://docs.rs/libsecp256k1/latest/libsecp256k1/ (if we wanted to generate a local per-module address/key-pair)