Add mnemonics for WalletClient

[DavidWz0403]

It would be great to add mnemonic initialization to the WalletClient in the foton library. This feature would allow users to import wallets using mnemonic phrases, derive key pairs, and sign transactions using those keys.

Requirements
Implement a method to initialize WalletClient with a mnemonic phrase.
Derive key pairs using a standard derivation path.
Enable transaction signing with the derived keys.