Use stream optional enum set from core in MLStatsInput

[q-andy]

Description

Refactors MLStatsInput to use StreamInput::readOptionalEnumSet and StreamOutput::writeOptionalEnumSet from OpenSearch core, added in this recent PR: https://github.com/opensearch-project/OpenSearch/pull/17556

The currently this is implemented in ml-commons using a private helper. I added this functionality to core using ml-commons as a reference so it could be reused in neural search, so the implementation is identical. While I made this change so other plugins could use the same function, I figure it might make sense to use it in ml-commons as well.

Compare ml-commons implmentation

private void writeEnumSet(StreamOutput out, EnumSet<?> set) throws IOException {
    if (set != null && set.size() > 0) {
        out.writeBoolean(true);
        out.writeEnumSet(set);
    } else {
        out.writeBoolean(false);
    }
}

Core implementation

public <E extends Enum<E>> void writeOptionalEnumSet(@Nullable EnumSet<E> enumSet) throws IOException {
    if (enumSet != null && enumSet.size() > 0) {
        writeBoolean(true);
        writeEnumSet(enumSet);
    } else {
        writeBoolean(false);
    }
}

Related Issues

n/a

Check List

• [ ] New functionality includes testing.
• [ ] New functionality has been documented.
• [ ] API changes companion pull request created.
• [x] Commits are signed per the DCO using --signoff.
• [ ] Public documentation issue/PR created.

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