Plugins: Requests validator (#30445)

* Introduce PluginRequestValidator abstraction with a NoOp implementation

* Update PluginRequestValidator abstraction to use the dsURL instead

* Inject PluginRequestValidator into the HTTPServer and validate requests going through data source proxy

* Inject PluginRequestValidator into the BackendPluginManager and validate requests going through it

* Validate requests going through QueryMetrics & QueryMetricsV2

* Validate BackendPluginManager health requests

* Fix backend plugins manager tests

* Validate requests going through alerting service

* Fix tests

* fix tests

* goimports

Co-authored-by: Leonard Gram <leo@xlson.com>

pkg/services/validations/oss.go

@@ -0,0 +1,15 @@
+package validations
+
+import (
+	"net/http"
+)
+
+type OSSPluginRequestValidator struct{}
+
+func (*OSSPluginRequestValidator) Init() error {
+	return nil
+}
+
+func (*OSSPluginRequestValidator) Validate(string, *http.Request) error {
+	return nil
+}