add mdoc expiration helper methods

[Ryanmtate]

This pull request introduces enhancements to the Mdoc struct in rust/src/credential/mdoc.rs, adding functionality to check expiration status and improving time-handling capabilities. The most important changes include the addition of two new accessor methods for expiration checks and the inclusion of a new time-related import.

Enhancements to Mdoc struct:

• Added is_mdoc_expired method to determine if the MSO (Mobile Security Object) has expired based on its valid_until field. This method uses the current UTC time for comparison.
• Added will_expire_in_num_days method to check if the MSO will expire within a specified number of days. This method calculates a target date using the current UTC time and the provided duration.

Time-handling improvements:

• Updated imports to include OffsetDateTime from the time crate, enabling precise time comparisons for the new methods.