Feature/fileinfo create copy (#9198)

* Initial implementation of a CopyFileInfos function that creates new FileInfo objects copied from provided FileIds with the provided user as the creator and not linked to a post yet. This can subsequently be used to copy existing attachments from another post to attach to a new post without having to re-upload the actual files

* added a unit test for the CopyFileInfos function

* resolving pull request suggestions

plugin/api.go

@@ -155,6 +155,13 @@ type API interface {
 	// UpdatePost updates a post.
 	UpdatePost(post *model.Post) (*model.Post, *model.AppError)
 
+	// CopyFileInfos creates a copy of FileInfo objects provided in the list of FileIds
+	// these new FileInfo objects will not be linked to any post and will
+	// be ready to provide to a new CreatePost call
+	// this should be used when you want to create a copy of a post including
+	// file attachments without duplicating the file uploads
+	CopyFileInfos(userId string, fileIds []string) ([]string, *model.AppError)
+
 	// KVSet will store a key-value pair, unique per plugin.
 	KVSet(key string, value []byte) *model.AppError