Fix parsing of DateHistogramBucket key

[Xtansia]

Description

Fixes the parsing of DateHistogramBuckets. The key was incorrectly being read as a long, when it should be a double. As can be seen here, the type parameter to KeyedBucket is double: https://github.com/opensearch-project/opensearch-net/blob/main/src/OpenSearch.Client/Aggregations/Bucket/DateHistogram/DateHistogramBucket.cs#LL34C51-L34C51
And response body from server looks like so:

{
    "_shards": {
        "failed": 0,
        "skipped": 0,
        "successful": 1,
        "total": 1
    },
    "aggregations": {
        "histogram#aggregation_ranges": {
            "buckets": [
                {
                    "doc_count": 1,
                    "key": 1672871085000.0,
                    "key_as_string": "2023-01-04T22:24:45.000Z"
                }
            ]
        }
    },
    "hits": {
        "hits": [],
        "max_score": null,
        "total": {
            "relation": "eq",
            "value": 1
        }
    },
    "timed_out": false,
    "took": 90
}

Issues Resolved

Fixes #130

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[Yury-Fridlyand]

Would you like adding query from #130 as a test?

[Xtansia]

Would you like adding query from #130 as a test?

@Yury-Fridlyand Sure, would you rather a full integration test, or just unit testing the deserialization using a mocked client & fixed response?

[Yury-Fridlyand]

I see IT is better and easier to add.

[Yury-Fridlyand]

@Xtansia, perhaps you've lost the fix.
BTW, the test doesn't fail now, so it doesn't validate the fix.

[Xtansia]

@Yury-Fridlyand Was intentional as I was trying to get the reproduce working correctly, have got the reproduce and fix properly pushed now