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>

modules/structs/pull.go

@@ -95,3 +95,16 @@ type EditPullRequestOption struct {
 	RemoveDeadline      *bool      `json:"unset_due_date"`
 	AllowMaintainerEdit *bool      `json:"allow_maintainer_edit"`
 }
+
+// ChangedFile store information about files affected by the pull request
+type ChangedFile struct {
+	Filename         string `json:"filename"`
+	PreviousFilename string `json:"previous_filename,omitempty"`
+	Status           string `json:"status"`
+	Additions        int    `json:"additions"`
+	Deletions        int    `json:"deletions"`
+	Changes          int    `json:"changes"`
+	HTMLURL          string `json:"html_url,omitempty"`
+	ContentsURL      string `json:"contents_url,omitempty"`
+	RawURL           string `json:"raw_url,omitempty"`
+}