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/authntest/fake.go

@@ -1,7 +1,22 @@
 package authntest
 
-import "github.com/grafana/grafana/pkg/services/authn"
+import (
+	"context"
+
+	"github.com/grafana/grafana/pkg/services/authn"
+)
 
 type FakeService struct {
 	authn.Service
 }
+
+var _ authn.Client = new(FakeClient)
+
+type FakeClient struct {
+	ExpectedErr      error
+	ExpectedIdentity *authn.Identity
+}
+
+func (f *FakeClient) Authenticate(ctx context.Context, r *authn.Request) (*authn.Identity, error) {
+	return f.ExpectedIdentity, f.ExpectedErr
+}