API endpoint for changing/creating/deleting multiple files (#24887)

This PR creates an API endpoint for creating/updating/deleting multiple
files in one API call similar to the solution provided by
[GitLab](https://docs.gitlab.com/ee/api/commits.html#create-a-commit-with-multiple-files-and-actions).

To archive this, the CreateOrUpdateRepoFile and DeleteRepoFIle functions
in files service are unified into one function supporting multiple files
and actions.

Resolves #14619

routers/api/v1/swagger/options.go

@@ -116,6 +116,9 @@ type swaggerParameterBodies struct {
 	// in:body
 	EditAttachmentOptions api.EditAttachmentOptions
 
+	// in:body
+	ChangeFilesOptions api.ChangeFilesOptions
+
 	// in:body
 	CreateFileOptions api.CreateFileOptions