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
"c": {
                "i": 3,
                "d": 0.3,
                "s": "ccc"
              }
            }""";
        try (XContentParser parser = createParser(JsonXContent.jsonXContent, content)) {
            XContentParseException exception = expectThrows(
                XContentParseException.class,
                () -> parser.map(HashMap::new, SimpleStruct::fromXContent)
            );
            assertThat(exception, hasMessage(containsString("s doesn't support values of type: VALUE_NUMBER")));
        }
    }
    public void testSubParserObject() throws IOException {
        XContentBuilder builder = XContentFactory.jsonBuilder();
        int numberOfTokens;
        numberOfTokens = generateRandomObjectForMarking(builder);
        String content = Strings.toString(builder);
        try (XContentParser parser = createParser(JsonXContent.jsonXContent, content)) {
            assertEquals(XContentParser.Token.START_OBJECT, parser.nextToken());
            assertEquals(XContentParser.Token.FIELD_NAME, parser.nextToken()); 
            assertEquals("first_field", parser.currentName());
            assertEquals(XContentParser.Token.VALUE_STRING, parser.nextToken()); 
            assertEquals(XContentParser.Token.FIELD_NAME, parser.nextToken()); 
            assertEquals("marked_field", parser.currentName());
            assertEquals(XContentParser.Token.START_OBJECT, parser.nextToken()); 
            XContentParser subParser = new XContentSubParser(parser);
            try {
                int tokensToSkip = randomInt(numberOfTokens - 1);