lfs: do not require auth for public repositories

LFS has the `authenticated` field in the response for downloads of
objects to indicate that the object does not need authentication to
access it.

Spec: https://github.com/git-lfs/git-lfs/blob/master/docs/api/batch.md#successful-responses

Fixes #23360.

app/controllers/projects/lfs_api_controller.rb

@@ -48,6 +48,10 @@ class Projects::LfsApiController < Projects::GitHttpClientController
     objects.each do |object|
       if existing_oids.include?(object[:oid])
         object[:actions] = download_actions(object)
+
+        if Guest.can?(:download_code, project)
+          object[:authenticated] = true
+        end
       else
         object[:error] = {
           code: 404,

changelogs/unreleased/lfs-noauth-public-repo.yml

+---
+title: Support unauthenticated LFS object downloads for public projects
+merge_request: 8824
+author: Ben Boeckel

spec/requests/lfs_http_spec.rb

@@ -600,6 +600,7 @@ describe 'Git LFS API and storage' do
             expect(json_response).to eq('objects' => [
               { 'oid' => sample_oid,
                 'size' => sample_size,
+                'authenticated' => true,
                 'actions' => {
                   'download' => {
                     'href' => "#{project.http_url_to_repo}/gitlab-lfs/objects/#{sample_oid}",