Add a blobs_only boolean parameter /projects/:id/repository/tree REST API endpoint

Description

This is a follow-up issue from Remove pagination from recursive form of /proje... (#365996 - closed)

It represents the following item which was discussed in this comment:

1. Add a blobs_only boolean parameter (will save ~17% of request payload size in the case of GitLab repo). This would require us to omit the -t (tree entries) option from the Gitaly ls-files call.