Re-add OnDiskGraphIndex::getDimension

[michaeljmarshall]

This getDimension method was removed from the OnDiskGraphIndex class via #536. We use it in C*, so I propose adding it back in since it is a getter on a final field.

[github-actions]

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[michaeljmarshall]

@marianotepper - I ran mvn clean test locally and all (unit?) tests passed, but it appears I do not have permission to retry the failure, which appears unrelated.

[MarkWolters]

@michaeljmarshall reran the failed tests this morning and everything passed. I'm not sure why a unit test failed initially but it doesn't appear to have anything to do with the content of this PR

[marianotepper]

Moving forward, OnDiskGraphIndex should not be used externally anymore. The interface ImmutableGraphIndex should be used exclusively, following #534. We can add the getDimension method to ImmutableGraphIndex, which would be equivalent to the requested change.

[marianotepper]

LGTM

[jshook]

LGTM