Getting a token from a container

[fitzthum]

The ASR does allow a container to request a token, but we haven't used this feature much and it could be kind of broken. For one, we can't specify the TEE public key. Without this the uses of the token are somewhat limited. On the other hand, it could be dangerous to allow setting the public key. That is basically an evidence factory attack and should only be used in certain cases. We should think about the best way to implement.

Also, our tooling in Kata doesn't know anything about the token option (see here). We should probably try to improve the documentation a little bit.