Use new IntDate type from jose-jwt

Replaces TokenTime.
Support for client_assertion authentication

- Adds supported auth method to Client data type
- Implements client_secret_jwt authentication

Still need to add private_key_jwt, which requires that clients can
register keys with the server.
private_key_jwt client authentication

- Adds storage of client keys during registration
- Retrieves keys from registered jwks_uri if needed
Make IdToken fields strict
Change APIs to match jose-jwt changes

- Change API to allow use of CPRG, in line with jose-jwt API changes to
  add blinding to RSA code
- Move client authentication to separate module
- Move client authentication back out of token processing function to
  make testing easier.
Bump version of jose-jwt dependency

Needed for API changes.