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/app.go

@@ -10,10 +10,10 @@ import (
 	"time"
 
 	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/public/shared/httpservice"
 	"github.com/mattermost/mattermost/server/public/shared/mlog"
 	"github.com/mattermost/mattermost/server/public/shared/timezones"
 	"github.com/mattermost/mattermost/server/v8/einterfaces"
-	"github.com/mattermost/mattermost/server/v8/platform/services/httpservice"
 	"github.com/mattermost/mattermost/server/v8/platform/services/imageproxy"
 	"github.com/mattermost/mattermost/server/v8/platform/services/searchengine"
 	"github.com/mattermost/mattermost/server/v8/platform/shared/templates"