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/models/validations.go

@@ -0,0 +1,12 @@
+package models
+
+import (
+	"net/http"
+)
+
+type PluginRequestValidator interface {
+	// Validate performs a request validation based
+	// on the data source URL and some of the request
+	// attributes (headers, cookies, etc).
+	Validate(dsURL string, req *http.Request) error
+}