Miden Wallet not able to retrieve private key

### Feature description

Add an option that allow us to retrieve private key/secret key for specific account instead of only seed phrase

### Why is this feature needed?

Currently there's no way for us to retrieve our private key for specific account and only seedphrase, in case we only need to import specific account its not doable right now

Look at the Miden wallet codebase for account management and key derivation. Find where seed phrases are used to generate accounts and where private keys are currently stored or derived. The goal is to add a method to export a specific account's private key, ensuring it aligns with the wallet's security model. Check for existing key management modules and any related tests to understand the current flow.

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