Auth: Add anonymous authn client (#59637)

* Authn: Add Client interface and Reqeust and Identity structures

* Authn: Implement Authenticate method in service

* Authn: Add tracing

* Authn: Add logger

* AuthN: Implement Anonymous client

pkg/services/authn/authnimpl/service_test.go

@@ -0,0 +1,62 @@
+package authnimpl
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/grafana/grafana/pkg/infra/log"
+	"github.com/grafana/grafana/pkg/infra/tracing"
+	"github.com/grafana/grafana/pkg/services/authn"
+	"github.com/grafana/grafana/pkg/services/authn/authntest"
+	"github.com/grafana/grafana/pkg/setting"
+)
+
+func TestService_Authenticate(t *testing.T) {
+	type TestCase struct {
+		desc        string
+		clientName  string
+		expectedErr error
+	}
+
+	tests := []TestCase{
+		{
+			desc:       "should succeed with authentication for configured client",
+			clientName: "fake",
+		},
+		{
+			desc:        "should fail when client is not configured",
+			clientName:  "gitlab",
+			expectedErr: authn.ErrClientNotFound,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			svc := setupTests(t, func(svc *Service) {
+				svc.clients["fake"] = &authntest.FakeClient{}
+			})
+
+			_, err := svc.Authenticate(context.Background(), tt.clientName, &authn.Request{})
+			assert.ErrorIs(t, tt.expectedErr, err)
+		})
+	}
+}
+
+func setupTests(t *testing.T, opts ...func(svc *Service)) *Service {
+	t.Helper()
+
+	s := &Service{
+		log:     log.NewNopLogger(),
+		cfg:     setting.NewCfg(),
+		clients: map[string]authn.Client{},
+		tracer:  tracing.InitializeTracerForTest(),
+	}
+
+	for _, o := range opts {
+		o(s)
+	}
+
+	return s
+}