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
}
        }
    }
    public void testReadBooleans() throws IOException {
        String falsy = randomFrom("\"false\"", "false");
        String truthy = randomFrom("\"true\"", "true");
        try (XContentParser parser = createParser(JsonXContent.jsonXContent, "{\"foo\": " + falsy + ", \"bar\": " + truthy + "}")) {
            XContentParser.Token token = parser.nextToken();
            assertThat(token, equalTo(XContentParser.Token.START_OBJECT));
            token = parser.nextToken();
            assertThat(token, equalTo(XContentParser.Token.FIELD_NAME));
            assertThat(parser.currentName(), equalTo("foo"));
            token = parser.nextToken();
            assertThat(token, in(Arrays.asList(XContentParser.Token.VALUE_STRING, XContentParser.Token.VALUE_BOOLEAN)));
            assertTrue(parser.isBooleanValue());
            assertFalse(parser.booleanValue());
            token = parser.nextToken();
            assertThat(token, equalTo(XContentParser.Token.FIELD_NAME));
            assertThat(parser.currentName(), equalTo("bar"));
            token = parser.nextToken();
            assertThat(token, in(Arrays.asList(XContentParser.Token.VALUE_STRING, XContentParser.Token.VALUE_BOOLEAN)));
            assertTrue(parser.isBooleanValue());
            assertTrue(parser.booleanValue());
        }
    }
    public void testEmptyList() throws IOException {
        XContentBuilder builder = XContentFactory.jsonBuilder().startObject().startArray("some_array").endArray().endObject();
        try (XContentParser parser = createParser(JsonXContent.jsonXContent, Strings.toString(builder))) {
            assertEquals(XContentParser.Token.START_OBJECT, parser.nextToken());
            assertEquals(XContentParser.Token.FIELD_NAME, parser.nextToken());