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/auth/join.go

@@ -125,6 +125,12 @@ func (a *Server) RegisterUsingToken(ctx context.Context, req *types.RegisterUsin
 			return nil, trace.Wrap(err)
 		}
 		joinAttributeSrc = claims
+	case types.JoinMethodGitLab:
+		claims, err := a.checkGitLabJoinRequest(ctx, req)
+		if err != nil {
+			return nil, trace.Wrap(err)
+		}
+		joinAttributeSrc = claims
 	case types.JoinMethodCircleCI:
 		claims, err := a.checkCircleCIJoinRequest(ctx, req)
 		if err != nil {
@@ -189,6 +195,7 @@ func (a *Server) generateCertsBot(
 		renewable = true
 	case types.JoinMethodIAM,
 		types.JoinMethodGitHub,
+		types.JoinMethodGitLab,
 		types.JoinMethodCircleCI,
 		types.JoinMethodKubernetes,
 		types.JoinMethodAzure: