JSON RPC function to add address from private key

[rraallvv]

New issue checklist

• I have read all of the README
• I have searched existing issues and this is not a duplicate.

Question or Feature Request

Function to add an address to the wallet using the JSON RPC API from the private key. The function could be like this:

    async addAccount(hex) {
        const entropy = new Nimiq.Entropy(Nimiq.BufferUtils.fromBase64(hex));
        const privateKey = new Nimiq.PrivateKey(entropy.serialize());
        const keyPair = Nimiq.KeyPair.derive(privateKey);
        const wallet = new Nimiq.Wallet(keyPair);
        await this._walletStore.put(wallet);
        return this._walletToObj(wallet);
    }

This is useful when moving the backend to a different server where there is already a JSON RPC node running, and therefore creating a new address in the new server could break the application on the client side.