Add API endpoint to get changed files of a PR (#21177)

This adds an api endpoint `/files` to PRs that allows to get a list of changed files.

built upon #18228, reviews there are included
closes https://github.com/go-gitea/gitea/issues/654

Co-authored-by: Anton Bracke <anton@ju60.de>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/api/v1/api.go

@@ -1002,6 +1002,7 @@ func Routes(ctx gocontext.Context) *web.Route {
 						m.Get(".{diffType:diff|patch}", repo.DownloadPullDiffOrPatch)
 						m.Post("/update", reqToken(), repo.UpdatePullRequest)
 						m.Get("/commits", repo.GetPullRequestCommits)
+						m.Get("/files", repo.GetPullRequestFiles)
 						m.Combo("/merge").Get(repo.IsPullRequestMerged).
 							Post(reqToken(), mustNotBeArchived, bind(forms.MergePullRequestForm{}), repo.MergePullRequest).
 							Delete(reqToken(), mustNotBeArchived, repo.CancelScheduledAutoMerge)