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
server/src/test/java/org/elasticsearch/index/mapper/DocumentParserTests.java
public void testDynamicBigInteger() throws Exception {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> {
            b.startArray("dynamic_templates");
            {
                b.startObject();
                {
                    b.startObject("big-integer-to-keyword");
                    {
                        b.field("match", "big-*");
                        b.field("match_mapping_type", "long");
                        b.startObject("mapping").field("type", "keyword").endObject();
                    }
                    b.endObject();
                }
                b.endObject();
            }
            b.endArray();
        }));
        BigInteger value = BigInteger.valueOf(Long.MAX_VALUE).add(BigInteger.ONE);
        ParsedDocument doc = mapper.parse(source(b -> b.field("big-integer", value)));
        List<IndexableField> fields = doc.rootDoc().getFields("big-integer");
        assertEquals(1, fields.size());
        assertEquals(new BytesRef(value.toString()), fields.get(0).binaryValue());
    }
    public void testDynamicBigDecimal() throws Exception {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> {
            b.startArray("dynamic_templates");
            {
                b.startObject();
                {