Introduce a new API for getting addresses from the Wallet that
reflects a similiar interface as the underlying indexer
`KeychainTxOutIndex` in preparation for removing `AddressIndex` enum.
Before this change, the only way to get an address was via the methods
`try_get{_internal}_address` which required a `&mut` reference to the
wallet, matching on the desired AddressIndex variant. This is too
restrictive since for example peeking or listing unused addresses
shouldn't change the state of the wallet. Hence we provide separate
methods for each use case which makes for a more efficient API.
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