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 cannot be an empty string"));
            }
            {
                DocumentParsingException e = expectThrows(
                    DocumentParsingException.class,
                    () -> mapper.parse(source(b -> b.field(" ", "foo")))
                );
                assertThat(e.getMessage(), containsString("Field name cannot contain only whitespace: [ ]"));
            }
            {
                DocumentParsingException e = expectThrows(
                    DocumentParsingException.class,
                    () -> mapper.parse(source(b -> b.startObject("foo").field("", "bar").endObject()))
                );
                assertThat(e.getCause(), notNullValue());
                assertThat(e.getCause().getMessage(), containsString("field name cannot be an empty string"));
            }
        });
    }
    public void testEmptyFieldNameSubobjectsFalse() throws Exception {
        DocumentMapper mapper = createDocumentMapper(mappingNoSubobjects(b -> {}));
        DocumentParsingException err = expectThrows(DocumentParsingException.class, () -> mapper.parse(source(b -> b.field("", "foo"))));
        assertThat(err.getCause(), instanceOf(IllegalArgumentException.class));
        assertThat(err.getCause().getMessage(), containsString("Field name cannot be an empty string"));
    }
    public void testBlankFieldNameSubobjectsFalse() throws Exception {
        DocumentMapper mapper = createDocumentMapper(mappingNoSubobjects(b -> {}));
        DocumentParsingException err = expectThrows(DocumentParsingException.class, () -> mapper.parse(source(b -> b.field("  ", "foo"))));
        assertThat(err.getMessage(), containsString("Field name cannot contain only whitespace: [  ]"));
    }