fix: handle single aud string claim, extract en/decoder interface, comments (#51)

* en/decoding abstraction

* some comments

* fix token validation and error messages

* fix: audience mapping (single aud string)

* fix tests with VerifyIdToken

* reformat imports

* go mod tidy

* Update pkg/oidc/authorization.go

Co-authored-by: Silvan <silvan.reusser@gmail.com>

* Update pkg/oidc/authorization.go

Co-authored-by: Silvan <silvan.reusser@gmail.com>

* Update pkg/op/authrequest_test.go

Co-authored-by: Silvan <silvan.reusser@gmail.com>

* fix capitalization

Co-authored-by: Silvan <silvan.reusser@gmail.com>

pkg/rp/mock/verifier.mock.go

@@ -0,0 +1,65 @@
+// Code generated by MockGen. DO NOT EDIT.
+// Source: github.com/caos/oidc/pkg/rp (interfaces: Verifier)
+
+// Package mock is a generated GoMock package.
+package mock
+
+import (
+	context "context"
+	oidc "github.com/caos/oidc/pkg/oidc"
+	gomock "github.com/golang/mock/gomock"
+	reflect "reflect"
+)
+
+// MockVerifier is a mock of Verifier interface
+type MockVerifier struct {
+	ctrl     *gomock.Controller
+	recorder *MockVerifierMockRecorder
+}
+
+// MockVerifierMockRecorder is the mock recorder for MockVerifier
+type MockVerifierMockRecorder struct {
+	mock *MockVerifier
+}
+
+// NewMockVerifier creates a new mock instance
+func NewMockVerifier(ctrl *gomock.Controller) *MockVerifier {
+	mock := &MockVerifier{ctrl: ctrl}
+	mock.recorder = &MockVerifierMockRecorder{mock}
+	return mock
+}
+
+// EXPECT returns an object that allows the caller to indicate expected use
+func (m *MockVerifier) EXPECT() *MockVerifierMockRecorder {
+	return m.recorder
+}
+
+// Verify mocks base method
+func (m *MockVerifier) Verify(arg0 context.Context, arg1, arg2 string) (*oidc.IDTokenClaims, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "Verify", arg0, arg1, arg2)
+	ret0, _ := ret[0].(*oidc.IDTokenClaims)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// Verify indicates an expected call of Verify
+func (mr *MockVerifierMockRecorder) Verify(arg0, arg1, arg2 interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "Verify", reflect.TypeOf((*MockVerifier)(nil).Verify), arg0, arg1, arg2)
+}
+
+// VerifyIDToken mocks base method
+func (m *MockVerifier) VerifyIDToken(arg0 context.Context, arg1 string) (*oidc.IDTokenClaims, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "VerifyIDToken", arg0, arg1)
+	ret0, _ := ret[0].(*oidc.IDTokenClaims)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// VerifyIDToken indicates an expected call of VerifyIDToken
+func (mr *MockVerifierMockRecorder) VerifyIDToken(arg0, arg1 interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "VerifyIDToken", reflect.TypeOf((*MockVerifier)(nil).VerifyIDToken), arg0, arg1)
+}