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.go

@@ -79,10 +79,10 @@ func (mr *MockAuthorizerMockRecorder) Encoder() *gomock.Call {
 }
 
 // IDTokenHintVerifier mocks base method.
-func (m *MockAuthorizer) IDTokenHintVerifier(arg0 context.Context) op.IDTokenHintVerifier {
+func (m *MockAuthorizer) IDTokenHintVerifier(arg0 context.Context) *op.IDTokenHintVerifier {
 	m.ctrl.T.Helper()
 	ret := m.ctrl.Call(m, "IDTokenHintVerifier", arg0)
-	ret0, _ := ret[0].(op.IDTokenHintVerifier)
+	ret0, _ := ret[0].(*op.IDTokenHintVerifier)
 	return ret0
 }