fix: resolve two TODO items

As far as I can tell, we're already doing item 7, and item 8 was a
one-line fix with no backcompat concerns

TODO.md

@@ -6,5 +6,3 @@ TODO
 - Remove the legacy package by folding all of its logic into the current package
 - Add an RPC test
 - Implement support for AgentSession.ID()
-- Endpoint.ID() should return the endpoint's API resource identifier but right now it just returns a random unique identifier unrelated to the API resource
-- Endpoint.Wait() which can be used to wait until an endpoint stops

endpoint.go

@@ -31,6 +31,9 @@ type Endpoint interface {
 	// Done returns a channel that is closed when the endpoint stops.
 	Done() <-chan struct{}
 
+	// Wait blocks until the endpoint stops.
+	Wait()
+
 	// ID returns the unique endpoint identifier assigned by the ngrok cloud service.
 	ID() string
 
@@ -86,6 +89,10 @@ func (e *baseEndpoint) Done() <-chan struct{} {
 	return e.doneChannel
 }
 
+func (e *baseEndpoint) Wait() {
+	<-e.doneChannel
+}
+
 func (e *baseEndpoint) ID() string {
 	return e.id
 }