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/main/java/org/elasticsearch/index/mapper/DocumentParserContext.java
}
        private State state = State.FIELD;
        private final String field;
        CopyToParser(String fieldName, XContentParser in) {
            super(in);
            this.field = fieldName;
            assert in.currentToken().isValue() || in.currentToken() == Token.VALUE_NULL;
        }
        @Override
        public Token nextToken() throws IOException {
            if (state == State.FIELD) {
                state = State.VALUE;
                return delegate().currentToken();
            }
            return Token.END_OBJECT;
        }
        @Override
        public Token currentToken() {
            if (state == State.FIELD) {
                return Token.FIELD_NAME;
            }
            return delegate().currentToken();
        }
        @Override
        public String currentName() throws IOException {
            return field;
        }
    }
}