Support privacy payment

[sunhuachuang]

Why we need privacy payment and assets ?

Because in real world, we donot want our payment information can be traced/seen by others.

If I am a customer, I donot want to share the balance with others, I donnot want to share the products transactions.

If I am a merchant, I also donot want to share how many products I sold and my customer infomation.

[sunhuachuang]

1. While privacy is good, but in real-world business, it would be even better if the privacy assets could be audited
2. The privacy assets can be integrating into the x402 protocol

TBD