[DISCUSS] PIP-424: Support Getting Message ID by Index in ManagedLedger Interface

[xiangying meng <xi...@apache.org>]

Hi everyone,
I would like to propose a new PIP to enhance the ManagedLedger
interface by adding support for getting a message ID by its index.
Currently, PIP-415 introduced the ability to get a message ID by index
using broker entry metadata. However, this approach has one main
drawback:

Custom ManagedLedger implementations (like StreamNative's Ursa engine)
may not use broker entry metadata for tracking message indexes.
I propose adding a new method to the ManagedLedger interface:

 ```java
 CompletableFuture<MessageId> getMessageIdByIndex(long messageIndex);
 ```

BR,
Xiangying Meng