client: refactor boot logic into SetUp method

This method only has heartbeat session population logic for now, so it's
actually a no-op during provisioning. However, there's probably some
value in "mandating" that clients call this shortly after construction,
so we have a chance to run any setup logic that we might need in the
future.

This _feels_ unidiomatic...?

pkg/connector/login_token.go

@@ -84,6 +84,7 @@ func (dl *DiscordTokenLogin) SubmitUserInput(ctx context.Context, input map[stri
 		connector: dl.connector,
 		Session:   session,
 	}
+	client.SetUp(ctx, nil)
 	err = client.connect(ctx)
 	if err != nil {
 		dl.softlyCloseSession()