BREAKING CHANGE:
- The various verifier types are merged into a oidc.Verifir.
- oidc.Verfier became a struct with exported fields
* use type aliases for oidc.Verifier
this binds the correct contstructor to each verifier usecase.
* fix: handle the zero cases for oidc.Time
* add unit tests to oidc verifier
* fix: correct returned field for JWTTokenRequest
JWTTokenRequest.GetIssuedAt() was returning the ExpiresAt field.
This change corrects that by returning IssuedAt instead.
