Potential memory leak when consuming for over a Month

https://github.com/googleapis/java-pubsublite-kafka/blob/4a575ea31c1abbf2576a08cebf5699986720e217/src/main/java/com/google/cloud/pubsublite/kafka/SingleSubscriptionConsumerImpl.java#L137

Hey java-pubsublite-kafka-team,

I noticed a potential memory leak in a heap dump of a long-running deployment of pubsublite-kafka). It occurred in a program which uses [spring-kafka](https://spring.io/projects/spring-kafka) in combination with pubsublite-kafka to consume a stream of (Avro encoded) data. It might be that we simply do not use the API correctly or have another issue. We use Maven and it does not indicate any conflicting dependencies for `pubsublite-kafka`.

**Problem**: After running and consuming data for *over a month*, the `wakeupTriggered` Future is reported at a size of over 6 GiB in the heap dump (after analyzing with [Eclipse MAT](https://www.eclipse.org/mat/)) for each Partition. The problem seems to be that `ApiFuturesExtensions.whenFirstDone` keeps adding Listeners but they are never GC-ed if the wakeupTrigger Future is never triggered. The analysis also shows that `assignmentChanged` has grown to a couple of MiBs because of Listeners, but it is nowhere near as big. I think it is being triggered every once in a while.

I’ve made a crude fix [in a fork](https://github.com/FransHeuvelmans/java-pubsublite-kafka/commit/0b673dc6955ddb6c5fc7e718b98f51dd408fc55d) but it kind of breaks the `wakeUp` functionality so it is probably not a good solution. The big memory leak was gone after this fix (for us it works).

Sadly I can’t share the code of the program and I haven’t invested time in a minimal bug recreation. I understand if this is too little information to go on but I thought I'd report it.

**Version info**:
pubsublite-kafka: v1.0.0
Spring-kafka: 2.9.0
libraries-bom: 26.0.0
Java version: openjdk 11.0.16 2022-07-19
GC: Garbage-First (G1) Garbage Collector