Support acknowledging a list of messages (#7688)

Fixes #7626


### Motivation
Expose `MessagesImpl` ,so that we can ack list of messages by using `CompletableFuture<Void> acknowledgeAsync(Messages<?> messages)`


### Modifications
Change the visibility level of the method from protect to public

### Verifying this change
unit test:
org.apache.pulsar.client.api.ConsumerBatchReceiveTest#testBatchAck

1 file changed