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
assertThat(
                e.getCause().getMessage(),
                containsString("field name starting or ending with a [.] makes object resolution ambiguous: [top. .foo]")
            );
        });
    }
    public void testDynamicFieldsEmptyName() throws Exception {
        dynamicTrueOrDynamicRuntimeTest(mapper -> {
            Exception exception = expectThrows(DocumentParsingException.class, () -> mapper.parse(source(b -> {
                b.startArray("top");
                {
                    b.startObject();
                    {
                        b.startObject("aoeu").field("a", 1).field(" ", 2).endObject();
                    }
                    b.endObject();
                }
                b.endArray();
            })));
            assertThat(exception.getMessage(), containsString("Field name cannot contain only whitespace: [top.aoeu. ]"));
        });
    }
    public void testBlankFieldNames() throws Exception {
        dynamicTrueOrDynamicRuntimeTest(mapper -> {
            {
                DocumentParsingException e = expectThrows(
                    DocumentParsingException.class,
                    () -> mapper.parse(source(b -> b.field("", "foo")))
                );
                assertThat(e.getCause(), notNullValue());