Introduce runtime section in mappings (#62906)

[javanna]

The runtime section is at the same level as the existing properties section. Its purpose is to hold runtime fields only. With the introduction of the runtime section, a runtime field can be defined by specifying its type (previously called runtime_type) and script.

PUT /my-index/_mappings
{
    "runtime" : {
        "day_of_week" : {
            "type" : "keyword",
            "script" : {
                "source" : "emit(doc['timestamp'].value.dayOfWeekEnum.getDisplayName(TextStyle.FULL, Locale.ROOT))"
            }
        }
    },
    "properties" : {
        "timestamp" : {
            "type" : "date"
        }
    }
}

Fields defined in the runtime section can be updated at any time as they are not present in the lucene index. They get replaced entirely when they get updated.

Thanks to the introduction of the runtime section, runtime fields override existing mapped fields defined with the same name, similarly to runtime fields defined in the search request.

Relates to #59332