Move HTTP service to public for plugin use (#27284)

* Move httpservice for use in plugins

* Adapt httpservice for plugin use

* Fix lint

server/channels/app/post_metadata_test.go

@@ -28,9 +28,9 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/public/shared/httpservice"
 	"github.com/mattermost/mattermost/server/v8/channels/app/platform"
 	"github.com/mattermost/mattermost/server/v8/channels/utils/testutils"
-	"github.com/mattermost/mattermost/server/v8/platform/services/httpservice"
 	"github.com/mattermost/mattermost/server/v8/platform/services/imageproxy"
 )