MINOR: UnifiedLog topic-partition as attribute as it is stable

[jeqo]

TopicPartition; different from e.g. LogConfig which can change and it's managed by LocalLog; is stable and can be its own attribute instead of accessing LocalLog#topicPartition

[github-actions]

A label of 'needs-attention' was automatically added to this PR in order to raise the
attention of the committers. Once this issue has been triaged, the triage label
should be removed to prevent this automation from happening again.

[github-actions]

A label of 'needs-attention' was automatically added to this PR in order to raise the
attention of the committers. Once this issue has been triaged, the triage label
should be removed to prevent this automation from happening again.

[github-actions]

This PR is being marked as stale since it has not had any activity in 90 days. If you
would like to keep this PR alive, please leave a comment asking for a review. If the PR has
merge conflicts, update it with the latest from the base branch.

If you are having difficulty finding a reviewer, please reach out on the [mailing list](https://kafka.apache.org/contact).

If this PR is no longer valid or desired, please feel free to close it. If no activity occurs in the next 30 days, it will be automatically closed.

[github-actions]

A label of 'needs-attention' was automatically added to this PR in order to raise the
attention of the committers. Once this issue has been triaged, the triage label
should be removed to prevent this automation from happening again.