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/mock/authorizer.mock.impl.go

@@ -49,7 +49,7 @@ func ExpectEncoder(a op.Authorizer) {
 func ExpectVerifier(a op.Authorizer, t *testing.T) {
 	mockA := a.(*MockAuthorizer)
 	mockA.EXPECT().IDTokenHintVerifier(gomock.Any()).DoAndReturn(
-		func() op.IDTokenHintVerifier {
+		func() *op.IDTokenHintVerifier {
 			return op.NewIDTokenHintVerifier("", nil)
 		})
 }