Namadillo: IBC -> List all available tokens for IBC withdrawal

[emccorson]

Following on from #1210, we should list all available tokens in a user's account as assets for IBC withdrawal.

Mateusz has updated the indexer to return IBC balances in the usual balance query, with the response being something like:

[
  {
    "tokenAddress": "tnam1qy440ynh9fwrx8aewjvvmu38zxqgukgc259fzp6h",
    "balance": "999000000"
  },
  {
    "tokenAddress": "tnam1p5nnjnasjtfwen2kzg78fumwfs0eycqpecuc2jwz",
    "balance": "18"
  }
]

We just need to display these balances and decode the token addresses. Namada CLI can already decode addresses, so we can just follow whatever it does.