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/clients/anonymous_test.go

@@ -0,0 +1,66 @@
+package clients
+
+import (
+	"context"
+	"fmt"
+	"testing"
+
+	"github.com/grafana/grafana/pkg/infra/log"
+	"github.com/grafana/grafana/pkg/services/authn"
+	"github.com/grafana/grafana/pkg/services/org"
+	"github.com/grafana/grafana/pkg/services/org/orgtest"
+	"github.com/grafana/grafana/pkg/setting"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestAnonymous_Authenticate(t *testing.T) {
+	type TestCase struct {
+		desc string
+		org  *org.Org
+		cfg  *setting.Cfg
+		err  error
+	}
+
+	tests := []TestCase{
+		{
+			desc: "should success with valid org configured",
+			org:  &org.Org{ID: 1, Name: "some org"},
+			cfg: &setting.Cfg{
+				AnonymousOrgName: "some org",
+				AnonymousOrgRole: "Viewer",
+			},
+		},
+		{
+			desc: "should return error if any error occurs during org lookup",
+			err:  fmt.Errorf("some error"),
+			cfg: &setting.Cfg{
+				AnonymousOrgName: "some org",
+				AnonymousOrgRole: "Viewer",
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.desc, func(t *testing.T) {
+			c := Anonymous{
+				cfg:        tt.cfg,
+				log:        log.NewNopLogger(),
+				orgService: &orgtest.FakeOrgService{ExpectedOrg: tt.org, ExpectedError: tt.err},
+			}
+
+			identity, err := c.Authenticate(context.Background(), &authn.Request{})
+			if err != nil {
+				require.Error(t, err)
+				require.Nil(t, identity)
+			} else {
+				require.Nil(t, err)
+
+				assert.Equal(t, true, identity.IsAnonymous)
+				assert.Equal(t, tt.org.ID, identity.OrgID)
+				assert.Equal(t, tt.org.Name, identity.OrgName)
+				assert.Equal(t, tt.cfg.AnonymousOrgRole, string(identity.Role()))
+			}
+		})
+	}
+}