CWT improvements

[eelanagaraj]

Two small improvements:

• Adds TryFrom<BTreeMap> for ClaimsSet: checks that keys are valid for CWT (int or string only) while parsing a BTreeMap -> ClaimsSet
  • updated a unit test to use this
• Makes NumericDate conversion a feature (numericdate-conversion) so that use cases that don't require this do not need time and fmul_to_int dependencies.
  • Moves this into a new file numericdate_conversion.rs
  • Very open to a different name for the feature/module

Tested:

• imported this in another repo and manually checked that the numericdate feature works (toy example with numericdate handling, checked that error imports worked as expected)

[cobward]

Nit: I'd suggest renaming the feature to time, which in the future could encapsulate further time dependent functionalities such as automatic expiration checking.