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
XContentBuilder builder = XContentFactory.jsonBuilder();
        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());
            IllegalStateException exception = expectThrows(IllegalStateException.class, () -> new XContentSubParser(parser));
            assertEquals("The sub parser has to be created on the start of an object or array", exception.getMessage());
        }
    }
    public void testCreateRootSubParser() throws IOException {
        XContentBuilder builder = XContentFactory.jsonBuilder();
        int numberOfTokens = generateRandomObjectForMarking(builder);
        String content = Strings.toString(builder);
        try (XContentParser parser = createParser(JsonXContent.jsonXContent, content)) {
            assertEquals(XContentParser.Token.START_OBJECT, parser.nextToken());
            try (XContentParser subParser = new XContentSubParser(parser)) {
                int tokensToSkip = randomInt(numberOfTokens + 3);
                for (int i = 0; i < tokensToSkip; i++) {
                    
                    assertNotNull(subParser.nextToken());
                }
            }
            assertNull(parser.nextToken());
        }
    }
    /**
     * Generates a random object {"first_field": "foo", "marked_field": {...random...}, "last_field": "bar}
     *