feat: merge the verifier types

BREAKING CHANGE:

- The various verifier types are merged into a oidc.Verifir.
- oidc.Verfier became a struct with exported fields

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() *oidc.Verifier {
 			return op.NewIDTokenHintVerifier("", nil)
 		})
 }