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.go

@@ -0,0 +1,44 @@
+/*
+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 "github.com/gravitational/trace"
+
+type envGetter func(key string) string
+
+// IDTokenSource allows a GitLab ID token to be fetched whilst executing
+// within the context of a GitLab actions workflow.
+type IDTokenSource struct {
+	getEnv envGetter
+}
+
+func (its *IDTokenSource) GetIDToken() (string, error) {
+	tok := its.getEnv("TBOT_GITLAB_JWT")
+	if tok == "" {
+		return "", trace.BadParameter(
+			"TBOT_GITLAB_JWT environment variable missing",
+		)
+	}
+
+	return tok, nil
+}
+
+func NewIDTokenSource(getEnv envGetter) *IDTokenSource {
+	return &IDTokenSource{
+		getEnv,
+	}
+}