Remove the need for Token.js

[darahayes]

src/KeycloakToken.ts is a direct copy of the Token class within the keycloak-nodejs-connect library. We copied it because the Keycloak library does not expose that class directly.

However, I have found keycloak.grantManager.createGrant is the more correct way for us to parse and validate the token. If we use this inside the KeycloakSubscriptionHandler, then we can completely remove the Token class.