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
DocumentParsingException.class,
            () -> mapper.parse(source(b -> b.startArray("foo").value(0).value(1).endArray()))
        );
        assertThat(
            exception.getMessage(),
            containsString("mapping set to strict, dynamic introduction of [foo] within [_doc] is not allowed")
        );
    }
    public void testDynamicRuntimeLongArray() throws Exception {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> b.field("dynamic", "runtime")));
        ParsedDocument doc = mapper.parse(source(b -> b.startArray("foo").value(0).value(1).endArray()));
        assertEquals(0, doc.rootDoc().getFields("foo").size());
        RuntimeField foo = doc.dynamicMappingsUpdate().getRoot().getRuntimeField("foo");
        assertEquals("""
            {"foo":{"type":"long"}}""", Strings.toString(foo));
    }
    public void testDynamicRuntimeDoubleArray() throws Exception {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> b.field("dynamic", "runtime")));
        ParsedDocument doc = mapper.parse(source(b -> b.startArray("foo").value(0.25).value(1.43).endArray()));
        assertEquals(0, doc.rootDoc().getFields("foo").size());
        RuntimeField foo = doc.dynamicMappingsUpdate().getRoot().getRuntimeField("foo");
        assertEquals("""
            {"foo":{"type":"double"}}""", Strings.toString(foo));
    }
    public void testDynamicRuntimeStringArray() throws Exception {
        DocumentMapper mapper = createDocumentMapper(topMapping(b -> b.field("dynamic", "runtime")));
        ParsedDocument doc = mapper.parse(source(b -> b.startArray("foo").value("test1").value("test2").endArray()));
        assertEquals(0, doc.rootDoc().getFields("foo").size());
        RuntimeField foo = doc.dynamicMappingsUpdate().getRoot().getRuntimeField("foo");
        assertEquals("""