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/userinfo.go

@@ -14,7 +14,7 @@ type UserinfoProvider interface {
 	Decoder() httphelper.Decoder
 	Crypto() Crypto
 	Storage() Storage
-	AccessTokenVerifier(context.Context) AccessTokenVerifier
+	AccessTokenVerifier(context.Context) *AccessTokenVerifier
 }
 
 func userinfoHandler(userinfoProvider UserinfoProvider) func(http.ResponseWriter, *http.Request) {