Add API endpoint to request contents of multiple files simultaniously (#34139)

Adds an API POST endpoint under `/repos/{owner}/{repo}/file-contents`
which receives a list of paths and returns a list of the contents of
these files.

This API endpoint will be helpful for applications like headless CMS
(reference: https://github.com/sveltia/sveltia-cms/issues/198) which
need to retrieve a large number of files by reducing the amount of
needed API calls.

Close #33495

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/structs/repo_file.go

@@ -176,3 +176,8 @@ type FileDeleteResponse struct {
 	Commit       *FileCommitResponse        `json:"commit"`
 	Verification *PayloadCommitVerification `json:"verification"`
 }
+
+// GetFilesOptions options for retrieving metadate and content of multiple files
+type GetFilesOptions struct {
+	Files []string `json:"files" binding:"Required"`
+}