HIP-0002: Available Addresses

I believe [HIP-0002](https://github.com/handshake-org/HIPs/blob/master/HIP-0002.md) should be expanded and an endpoint listing available addresses should be implemented.

For example, a GET request to `https:///.well-known/wallets` could retrieve a list of supported currencies.

The response could simply be a comma-separated plain-text list. The currencies could either be represented with their symbol, such as `BTC,ETH,HNS`, or with their [SLIP-0044 coin type](url), such as `0,60,5353`.

Other blockchain naming services have [implemented similar features](https://docs.ens.domains/dapp-developer-guide/resolving-names#listing-cryptocurrency-addresses-and-text-records).