Merge oidc and oauth2 token helpers

[nsklikas]

There is a lot of code shared between https://github.com/IdentityPython/oidc-op/blob/master/src/oidcop/oauth2/token.py and https://github.com/IdentityPython/oidc-op/blob/master/src/oidcop/oidc/token.py. We should refactor the helper classes so that duplicate code is removed

[peppelinux]

Completely agree

Probably this Will affect also how unit tests imports things and this could be a bit Dangerous, because we should do refactor without touching unit tests ...