feat: allow to configure JWT leeway in open_id_connect and increase default to 1 second

[jirutka]

Proposed changes

The default value defined in the jwt module is 0 seconds, so when the time is off just by a few milliseconds, the token validation fails with error "The token is not yet valid (iat)".

Checklist

• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
  • This feature is already in the azuread_b2c and mediawiki backends, but there are no tests for it, so I don’t have any reference to based it on and don’t have time to figure it out myself.
• I have added documentation to https://github.com/python-social-auth/social-docs
  • I don’t see any documentation for the generic open_id_connect backend in https://github.com/python-social-auth/social-docs/tree/master/docs/backends.