Add JsonGenerator.writeStartObject(Object, int) (needed by CBOR, maybe Avro)

[cowtowncoder]

(for background, see: FasterXML/jackson-dataformats-binary#3)

Some binary data formats have optimized versions of element-prefixed Objects and/or Arrays.
For Arrays we already have writeStartArray(int), but for Objects there is no equivalent.

But instead of adding writeStartElement(int), I think we should add:

• writeStartObject(Object, int)

to pass databinding level value for which entries are written, possibly used by streaming generator.
And while doing that, also add:

• writeStartArray(Object, int)

for sake of completeness.

These can be added in 2.10, but probably not made use of until a later point (3.0, likely).

[cowtowncoder]

Both were added, but as per notes, should try to avoid use until 2.11 / 3.0