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
return parser.mapStrings();
        }
    }
    public void testReadBooleansFailsForLenientBooleans() throws IOException {
        String falsy = randomFrom("\"off\"", "\"no\"", "\"0\"", "0");
        String truthy = randomFrom("\"on\"", "\"yes\"", "\"1\"", "1");
        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_NUMBER)));
            assertFalse(parser.isBooleanValue());
            if (token.equals(XContentParser.Token.VALUE_STRING)) {
                expectThrows(IllegalArgumentException.class, parser::booleanValue);
            } else {
                expectThrows(XContentParseException.class, 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_NUMBER)));
            assertFalse(parser.isBooleanValue());
            if (token.equals(XContentParser.Token.VALUE_STRING)) {
                expectThrows(IllegalArgumentException.class, parser::booleanValue);
            } else {
                expectThrows(XContentParseException.class, parser::booleanValue);