[Enhancement] Support getting message index in `ManagedLedger` interface

### Search before reporting

- [x] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### Motivation

1. [PIP-415](https://github.com/apache/pulsar/pull/24220) supports getting a message ID by index. This capability relies on `broker entry metadata` to obtain the message index. However, custom message layer implementations (e.g., StreamNative's Ursa engine) may not utilize broker entry metadata for this purpose. see more details in https://github.com/apache/pulsar/pull/24222#issuecomment-2921951865
2. The current implement of searching message index is inefficient, that needs to traverse all entries and check whether their indexes meet the requirements.

### Solution

1. Add a method `findMessageIndexByPosition` to the `managerLedger` interface.
2. Record the first entry index in `LedgerInfo`—the manager ledger can directly compute the index based on the message ID, significantly improving API efficiency and avoiding read amplification.

### Alternatives

Add a method

### Anything else?

_No response_

### Are you willing to submit a PR?

- [x] I'm willing to submit a PR!