Store account keys securely

As a temporary solution we are storing the account key pair in the database unencrypted, but we need to decide on a more flexible and future-proof approach [as @bobbinth suggested](https://github.com/0xPolygonMiden/miden-client/pull/28#discussion_r1403027491). The main use-case we need to support now is to be able to provide authentication easily (but we will also probably want a way to import/export accounts in the future).

Look at how account keys are currently stored in the database. Research secure storage options like hardware security modules, encrypted key stores, or secure enclaves. The PR discussion linked in the issue body provides context. The goal is to design a system that supports authentication and future import/export, then implement it.

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