feat: merge the verifier types (#336)

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.

pkg/op/session.go

@@ -13,7 +13,7 @@ import (
 type SessionEnder interface {
 	Decoder() httphelper.Decoder
 	Storage() Storage
-	IDTokenHintVerifier(context.Context) IDTokenHintVerifier
+	IDTokenHintVerifier(context.Context) *IDTokenHintVerifier
 	DefaultLogoutRedirectURI() string
 }