varLengthStringDictionary outperforms fixedLengthStringDictionary for `dictionary.indexOf()`

https://github.com/apache/pinot/pull/10007

From the test above, `varLengthStringDictionary` outperforms `fixedSizeStringDictionary` for `dictionary.indexOf()` calls and this is not intuitive because varLengthDictionary has 1 extra level of indirection. We can investigate and understand why this is the case.

```
BenchmarkStringVarLengthDictionary.fixedStringDictionaryIndexOf avgt 5 2802.320 ± 308.715 ms/op
BenchmarkStringVarLengthDictionary.fixedStringDictionaryGet avgt 5 213.743 ± 15.349 ms/op
BenchmarkStringVarLengthDictionary.varLengthStringDictionaryIndexOf avgt 5 2079.673 ± 78.193 ms/op
BenchmarkStringVarLengthDictionary.varLengthStringDictionaryGet avgt 5 227.187 ± 13.771 ms/op
```

Start with linked PR #10007 and the BenchmarkStringVarLengthDictionary benchmark results in the issue. Compare the fixed-length and variable-length dictionary implementations around dictionary.indexOf() and dictionary.get(), then rerun the benchmark to confirm the behavior. Done means explaining why the variable-length dictionary is faster for indexOf() despite its extra indirection.

Written by the indexing model from the issue text.