Currently, when an object is mapped with `subobjects: false`, trying to ingest a document that has nested objects leads to a mapping exception.

In a couple of occasions, we've been talking about that it would be convenient if Elasticsearch could automatically flatten the fields instead of the client having to know about the `subobjects` setting. This would make `subobjects: false` more of a drop-in replacement that allows for a much easier adoption, without changes in the client.

Note that the `_source` should ideally stay untouched, just the stored fields should be flattened automatically.

Some related issues
* https://github.com/elastic/apm/issues/347
* https://github.com/elastic/elasticsearch/issues/88777
* https://github.com/elastic/package-spec/issues/349
libs/x-content/src/test/java/org/elasticsearch/xcontent/XContentParserTests.java
"i": 2,
                "d": 0.2,
                "s": "bbb"
              }
            }""";
        SimpleStruct structA = new SimpleStruct(1, 0.1, "aaa");
        SimpleStruct structB = new SimpleStruct(2, 0.2, "bbb");
        SimpleStruct structC = new SimpleStruct(3, 0.3, "ccc");
        try (XContentParser parser = createParser(JsonXContent.jsonXContent, content)) {
            Map<String, SimpleStruct> actualMap = parser.map(LinkedHashMap::new, SimpleStruct::fromXContent);
            
            assertThat(actualMap, equalTo(Map.of("a", structA, "b", structB, "c", structC)));
            
            assertThat(actualMap.values(), contains(structC, structA, structB));
            assertNull(parser.nextToken());
        }
    }
    public void testGenericMap_Failure_MapContainingUnparsableValue() throws IOException {
        String content = """
            {
              "a": {
                "i": 1,
                "d": 0.1,
                "s": "aaa"
              },
              "b": {
                "i": 2,
                "d": 0.2,
                "s": 666
              },