[MM-53454] Add export file settings + slash command for public link (#23915)

Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/file.go

@@ -3,6 +3,8 @@
 
 package model
 
+import "time"
+
 const (
 	MaxImageSize = int64(6048 * 4032) // 24 megapixels, roughly 36MB as a raw image
 )
@@ -11,3 +13,8 @@ type FileUploadResponse struct {
 	FileInfos []*FileInfo `json:"file_infos"`
 	ClientIds []string    `json:"client_ids"`
 }
+
+type PresignURLResponse struct {
+	URL        string        `json:"url"`
+	Expiration time.Duration `json:"expiration"`
+}