Add `writeStartObject(Object, int)` in generator API to support fixed-element writes (CBOR et al)

Currently there are methods for indicating element count for arrays to write, but not Objects.
But underlying ability is expected and exposed by some codecs, such as CBOR; in some cases just as optimization (indicating length can be more compact and/or allow better decoding), and in other cases possibly as a requirement.
Regardless, it would make sense to expose this at general generator level (`JsonGenerator`).