Transport Backpressure support

[marandaneto]

Similar to Java https://github.com/getsentry/sentry-java/blob/main/sentry/src/main/java/io/sentry/transport/QueuedThreadPoolExecutor.java

The idea is that events get queued up to X number and dropped if the limit gets exceeded, so we don't overload the app's main isolate if they capture events very fast in a tight loop.

[marandaneto]

We should do it to avoid issues when calling e.g. captureException in a tight loop, since it returns a Future and the throwable is not serializable, we cannot offload the event pipeline in a different Isolate, we could tho try to use a counter and bailout if the queue is exceeded.
Also, you cannot call MethodChannels in the main isolate.

[marandaneto]

Consider doing #536 as well