GitLab Delegated Joining (#22705)

* Add type for GitLab ProvisionToken

* Add default behaviour for domain

* Add IDTokenClaims for GitLab

* Add gitlab token source and token validator

* Thread GitLab support through auth and tbot packages

* Adjust cluster name fetching in token validator

* Initialize GitLab token validator in auth

* Improve comment on `sub`

* Working GitLab CI delegated joining

* Add additional token rule fields

* Add checking for new configuration fields

* add additional test cases for validation of gitlab config struct

* Add TestAuth_RegisterUsingToken_GitLab

* Add tests for IDTokenSource

* Fix imports

* Add tests for GitLab Token Validator

* Fix some comments that were incomplete

* Add license headers

lib/gitlab/token_source_test.go

@@ -0,0 +1,52 @@
+/*
+Copyright 2023 Gravitational, Inc.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package gitlab
+
+import (
+	"testing"
+
+	"github.com/gravitational/trace"
+	"github.com/stretchr/testify/require"
+)
+
+func TestIDTokenSource_GetIDToken(t *testing.T) {
+	t.Run("value present", func(t *testing.T) {
+		its := &IDTokenSource{
+			getEnv: func(key string) string {
+				if key == "TBOT_GITLAB_JWT" {
+					return "foo"
+				}
+				return ""
+			},
+		}
+		tok, err := its.GetIDToken()
+		require.NoError(t, err)
+		require.Equal(t, "foo", tok)
+	})
+
+	t.Run("value missing", func(t *testing.T) {
+		its := &IDTokenSource{
+			getEnv: func(key string) string {
+				return ""
+			},
+		}
+		tok, err := its.GetIDToken()
+		require.Error(t, err)
+		require.True(t, trace.IsBadParameter(err))
+		require.Equal(t, "", tok)
+	})
+}