Plugins: Automatically forward plugin request HTTP headers in outgoing HTTP requests (#60417)

Automatically forward core plugin request HTTP headers in outgoing HTTP requests. 
Core datasource plugin authors don't have to specifically handle forwarding of HTTP 
headers, e.g. do not have to "hardcode" the header-names in the datasource plugin, 
if not having custom needs.

Fixes #57065

pkg/services/pluginsintegration/pluginsintegration.go

@@ -81,5 +81,7 @@ func CreateMiddlewares(cfg *setting.Cfg, oAuthTokenService oauthtoken.OAuthToken
 		middlewares = append(middlewares, clientmiddleware.NewUserHeaderMiddleware())
 	}
 
+	middlewares = append(middlewares, clientmiddleware.NewHTTPClientMiddleware())
+
 	return middlewares
 }