[Feature request] Time slack and default timedelta in oidcop configuration

[peppelinux]

https://github.com/IdentityPython/oidc-op/pull/155/files#diff-fdd667e49ac1308c4888bce310067fbe8ffeee845186585dea4533179bca562fL228

we also need a timeslack paramenter (an integer in seconds) in the general configuration of an entity to have a more relaxed validation of exp