Add filesize to response payload of /projects/:id/repository/tree REST API endpoint
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## Description
This is a follow-up issue from https://gitlab.com/gitlab-org/gitlab/-/issues/365996+
It represents the following item which was discussed in [this comment](https://gitlab.com/gitlab-org/gitlab/-/issues/355019#note_1024388263):
1. Add `filesize` to response payload, since it is easy to obtain via the `-l` option to the Gitaly `ls-files` call.
issue