Allow Züs-only allocations on Vult to be purchased with ZCN

[rikachet1]

Currently, users can only purchase Zus-only blobbers (through Vult/Blimp UI) with USD/Stripe. This hinders prevents privacy-conscious users from access a set of reliable blobbers. Using ZCN would bring back the token price and improve the tokenomics.

The primary goal would be to :

• BE: Automatically set the ZCN price for Züs-only allocation/blobbers, so it would have similar price as its USD amount. I.e if ZCN is at $0.0054, then the ZCN cost for a 50GB that costs $0.99/month would be around 1868ZCN. Potentially, we could give a slight discount when buying with ZCN instead of with USD. This can incentives people to use ZCN.
• FE : When purchasing or upgrading storage on the Vult Webapp, the "Pay with ZCN" should be available
  This also ties into the mnemonic-only login option, but for now this would just be an option for those who have existing wallets

Note: Currently Zboxcli allows you to create allocations with zus-only blobbers with ZCN.