Known token infos

[ImplOfAnImpl]

When a token transfer is being signed, the way the amount being transferred is displayed has to depend on whether the token is known:

1. If a token is known, we can just use the decimal amount and the token ticker, e.g. "1.5 USDT" (we should be able to assume that the token ticker is unique among known tokens).
2. If the token is unknown, neither the ticker not the number of decimals can be relied upon, so we have to mention that the token is unknown and display the token id as well as the amount in atoms.

For now we treat any token as unknown.

So,

• We need a list of known token infos hardcoded into the firmware itself.
• We also need a way of receiving reliable token infos from the host, so that adding a new token doesn't require updating the firmware.
  (For the infos to be reliable, they have to be signed by a known key. The host software may then either have the signed infos hardcoded or obtain them from some endpoint).