* 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>