Support for render metadata in credential offer

[Exulansis]

Description

In order to allow for better rendering / UX of credential offers on client devices (e.g. mobile wallets), issuers must be able to provide metadata for styling purposes (e.g. logo).

Updating the requestedInput field is not in scope for this issue.

To be done:

• [x] Define data structure for credential offers (current status)
• [x] Define data structure for credential offer responses
• [x] Update the interaction tokens according to the new data structure (requestedInput acts as a placeholder for now)
• [x] Adjust tests
• [x] Document the new flows

Note: The current data structure for the renderInfo field is meant to be compatible with the credential manifest as defined by the DIF. At later points in time, the @context field can be introduced to normalize the key names

[Exulansis]

Last thing left for now is documentation.

[Exulansis]

Documentation has been added, section 5 updated.