Allow format sort values of date fields

[dnhatn]

Backport of #70357

If a search after request targets multiple indices and some of its sort
field has type date in one index but date_nanos in other indices,
then Elasticsearch won't interpret the search_after parameter correctly
in every target index. The sort value of a date field by default is a
long of milliseconds since the epoch while a date_nanos field is a long
of nanoseconds.

This commit introduces the format parameter in the sort field so a
sort value of a date or date_nanos will be formatted using a date format
in a search response.

The below example illustrates how to use this new parameter.

{
    "query": {
        "match_all": {}
    },
    "sort": [
        {
            "timestamp": {
                "order": "asc",
                "format": "strict_date_optional_time_nanos"
           }
        }
    ]
}

{
    "query": {
        "match_all": {}
    },
    "sort": [
        {
            "timestamp": {
                "order": "asc",
                "format": "strict_date_optional_time_nanos"
            }
        }
    ],
    "search_after": [
        "2015-01-01T12:10:30.123456789Z" // in `strict_date_optional_time_nanos` format
    ]
}

Closes #69192