Check valid offload storage at startup

### Search before asking

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

### Motivation

If storage offload is configured, Pulsar broker should check at startup whether the storage bucket is accessible, and print an error message if it is not. This alerts the user when starting up a Pulsar cluster with a broken offload configuration.

### Solution

Add storage offload check at startup. Print an error message if there is any problem writing to the storage bucket.

### Alternatives

_No response_

### Anything else?

no

### Are you willing to submit a PR?

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

The issue identifies no files, entry points, or tests. Start by locating Pulsar broker startup handling and storage-offload configuration, then verify that startup reports an inaccessible bucket and covers problems writing to it.

Written by the indexing model from the issue text.