Add `justification` field

[danfinlay]

A requested permission should permit an optional justification field that is passed to the user confirmation function, to enhance the context of the prompt.

Should support text, but also be forward-extensible to other justification formats.

[rekmarks]

This feature would be problematic from a localization perspective, but is perhaps something we could consider if we ever have programmatic (i.e. LLM-based) translations in the wallet.