ICRC-21: Improve user preferences to be able to handle line displays

[frederikrothenberger]

This PR refactors how the user preferences are handled in the ICRC-21 standard:

• There is now a single icrc21_consent_message_spec present in both the request and the response. This makes it easier for signers to compare the requested spec to the actual spec.
• The icrc21_consent_message_spec can now be expanded without affecting other types (previously the language preference was duplicated in the icrc21_consent_preferences and icrc21_consent_info).
• The icrc21_consent_message_spec is expanded to handle line displays. This is required for devices such as thee Ledger hardware wallets to process ICRC-21 consent messages.

The candid interface is split into a separate file to make it easier to use for developers.

[frederikrothenberger]

@sea-snake: I made the device_spec optional in the icrc21_consent_message_spec to address the points raised in working group meeting that we had earlier today. I also clarified the comments to be more explicit about fallbacks.