Strict typescript compliance

[timechainz]

Optional members on TokenInterface breaks projects that want to enforce strict typescript compliance.

Modify Json type to support undefined and explicitly override the optional properties on TokenInterface to support undefined type.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}