Implement LNProxy-like privacy

[jooray]

Wallet providers with outsized view of the network (custodial wallets such as Wallet of Satoshi, or providers that handle routing, such as Phoenix Wallet -> acinq) can determine a btcpayserver's e-shop turnover and number of transactions by scanning and saving public keys. There is no need to give these providers this information.

BTCPayServer could implement a mechanism similar to LNProxy to create random public keys and proxy the payments to make this type of surveillance a little bit more difficult.

https://github.com/lnproxy/lnproxy