Add Date and Date Time credential claim types

[swcurran]

Enable support for Date and DateTime encoding types in the credential schema so that ISO 8601-formatted Date and Date Time strings can be encoded as integers in either DateInt (Date) or UnixTime (DateTime) format. While UnixTime is well-known (typically, the number of seconds since Jan. 1, 1970), DateInt is less so. It is the conversion of a date into the YYYMMDD integer equivalent. For example, 2023.11.12 becomes 20231112 or 20,231,112.

Adding this also for human-friendly date strings to be put into a credentials, and to use them in predicates such as "older than 20" based on (ISO 8601 formatted) date of birth without sharing the date of birth.

[sroopsai]

Hello @swcurran can you please assign me for this issue.

[swcurran]

Feel free to document a design here for what you plan, or a PR to implement the feature. Some design is likely need to think through how this will work — I don’t think it is a trivial one. Not too bad from a coding perspective, I think — just the concepts have to be thought through.