List token on OpenDEX

[ghost]

This issue is about enabling users to list arbitrary ERC20 tokens on OpenDEX via xud. Connext relies on the router node holding the asset in order to collateralize inbound liquidity for non-routing nodes.

Proposed flow:

• xud periodically fetches all the supported tokens from Connext router node.
• when user attempts to call addcurrency for ERC20 token that is not supported xud will throw an error to indicate that the token is currently not listed. The user can list the token via xucli list-token 0xTokenAddress command.
• xucli list-token command hits http://routing.node/api/list-token/0xTokenAddress endpoint which returns the signer address of the routing node. The signer address is then monitored for new deposits for 0xTokenAddress. Once the routing node has enough funds the system will add the token to supported tokens configuration (and restart the node if necessary).