feat!: add a new function to generate and track unused addresses in ObservableWallets

[AngelCastilloB]

Context

Our CIP-30 getUnusedAddresses implementation currently always returns an empty array. Particular DApps require wallets to use a freshly created address, as such we must enable this feature in our CIP-30 implementation.

Proposed Solution

BaseWallet now implements a new getNextUnusedAddress method, which tries to find and track the next unused wallet, it does this by first listing all known addresses, picking the address with the highest derivation index and checking whether this is an empty address or not, if it is an unused address, it returns this address, otherwise, it generates a new unused address, adds it to the tracked addresses and returns it. This guarantees that we are always tracking our next unused address if it is ever requested.

Important Changes Introduced

• BaseWallet implementes a new getNextUnusedAddress method that returns the next empty address.
• CIP-30 getUnusedAddresses now return an array with the next unused address.