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
.endObject()
            .startObject()
            .endObject()
            .endArray()
            .endObject();
        try (XContentParser parser = createParser(JsonXContent.jsonXContent, Strings.toString(builder))) {
            assertEquals(XContentParser.Token.START_OBJECT, parser.nextToken());
            assertEquals(XContentParser.Token.FIELD_NAME, parser.nextToken());
            assertEquals("some_array", parser.currentName());
            if (random().nextBoolean()) {
                
                assertEquals(XContentParser.Token.START_ARRAY, parser.nextToken());
            }
            assertEquals(Arrays.asList(singletonMap("foo", "bar"), emptyMap()), parser.list());
        }
    }
    public void testGenericMap() throws IOException {
        String content = """
            {
              "c": {
                "i": 3,
                "d": 0.3,
                "s": "ccc"
              },
              "a": {
                "i": 1,
                "d": 0.1,
                "s": "aaa"
              },
              "b": {