feat(wallet)!: add new_or_load methods

These methods try to load wallet from persistence and initializes the
wallet instead if non-existant.

An internal helper method `create_signers` is added to reuse code.
Documentation is also improved.

example-crates/wallet_esplora_async/src/main.rs

@@ -2,6 +2,7 @@ use std::{io::Write, str::FromStr};
 
 use bdk::{
     bitcoin::{Address, Network},
+    chain::PersistBackend,
     wallet::{AddressIndex, Update},
     SignOptions, Wallet,
 };