Implement OFREP compatible feature flag service (#105632)

* Add ofrep pkg

* api server: Use namespace from request in case user is not authenticated

* Add handlers to ofrep api builder

* Add NewOpenFeatureService to initialize mt apiserver

* allow specifying CA and insecure

* Compare namespace with eval ctx stackID

* Organize ofrep package

* Implement AllowedV0Alpha1Resources

* Revert folderimpl changes

* Handle default namespace

* Fix extracting stack id from eval ctx

* Add more logs

* Update pkg/registry/apis/ofrep/register.go

Co-authored-by: Dave Henderson <dave.henderson@grafana.com>

* Update pkg/registry/apis/ofrep/register.go

Co-authored-by: Dave Henderson <dave.henderson@grafana.com>

* Apply review feedback

* Replace contexthandler with types

* Fix identifying authed request

* Refactor checks in the handlers

* Remove anonymous from isAuthenticatedRequest check

---------

Co-authored-by: Todd Treece <360020+toddtreece@users.noreply.github.com>
Co-authored-by: Gabriel Mabille <gabriel.mabille@grafana.com>
Co-authored-by: Charandas Batra <charandas.batra@grafana.com>
Co-authored-by: Dave Henderson <dave.henderson@grafana.com>

pkg/registry/apis/ofrep/static.go

@@ -0,0 +1,41 @@
+package ofrep
+
+import (
+	"net/http"
+
+	"github.com/grafana/grafana/pkg/services/featuremgmt"
+)
+
+func (b *APIBuilder) evalAllFlagsStatic(isAuthedUser bool, w http.ResponseWriter, r *http.Request) {
+	result, err := b.staticEvaluator.EvalAllFlags(r.Context())
+	if err != nil {
+		b.logger.Error("Failed to evaluate all static flags", "error", err)
+		http.Error(w, "failed to evaluate flags", http.StatusInternalServerError)
+		return
+	}
+
+	if !isAuthedUser {
+		var publicOnly []featuremgmt.OFREPFlag
+
+		for _, flag := range result.Flags {
+			if isPublicFlag(flag.Key) {
+				publicOnly = append(publicOnly, flag)
+			}
+		}
+
+		result.Flags = publicOnly
+	}
+
+	writeResponse(http.StatusOK, result, b.logger, w)
+}
+
+func (b *APIBuilder) evalFlagStatic(flagKey string, w http.ResponseWriter, r *http.Request) {
+	result, err := b.staticEvaluator.EvalFlag(r.Context(), flagKey)
+	if err != nil {
+		b.logger.Error("Failed to evaluate static flag", "key", flagKey, "error", err)
+		http.Error(w, "failed to evaluate flag", http.StatusInternalServerError)
+		return
+	}
+
+	writeResponse(http.StatusOK, result, b.logger, w)
+}