Add TermHandler

[fujiwara]

SIGTERM handler

ridge catches SIGTERM and stops the server gracefully by default.

You can add a custom handler to the SIGTERM signal.

r := ridge.New(":8080", "/", mux)
r.SIGTERMHandler = func() {
	// your custom handler
}
r.RunWithContext(ctx)

ridge calls the handler when it receives a SIGTERM signal. After the handler returns, ridge stops the server.

When you run ridge on AWS Lambda, ridge uses lambda.WithEnableSIGTERM to call the handler. In this case, the handler must return in 500ms.