`BismuthWallet` instance is created every time `BismuthClient` is instantiated

[Endogen]

In BismuthClient in the __init__()method a BismuthWallet is instantiated by calling self.load_wallet(wallet_file).

That is not necessary if there is no wallet.der file but a wallet.json file.

[hclivess]

I will add conversion from wallet.der to wallet.json to legacy wallet so we can remove this compatibility