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
builder.endArray();
        builder.endObject();
        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("array", parser.currentName());
            assertEquals(XContentParser.Token.START_ARRAY, parser.nextToken()); 
            XContentParser subParser = new XContentSubParser(parser);
            try {
                int tokensToSkip = randomInt(numberOfTokens);
                for (int i = 0; i < tokensToSkip; i++) {
                    
                    assertNotNull(subParser.nextToken());
                }
                if (randomBoolean()) {
                    
                    subParser.skipChildren();
                }
            } finally {
                assertFalse(subParser.isClosed());
                subParser.close();
                assertTrue(subParser.isClosed());
            }
            assertEquals(XContentParser.Token.END_ARRAY, parser.currentToken());
            assertEquals(XContentParser.Token.END_OBJECT, parser.nextToken());
            assertNull(parser.nextToken());
        }
    }
    public void testCreateSubParserAtAWrongPlace() throws IOException {