Add JsonGenerator.writeFieldId(long) method to support binary formats with non-String keys

[cowtowncoder]

Some/many binary formats allow use of keys other than Strings; CBOR for one. Since coercion from non-Strings is already needed even with JSON (to support Java Maps with typed keys), it would be easy to add a method like:

public writeFieldId(long id);

with default implementation that just converts id into String and calls writeFieldName(), and where dataformat implementations may override it. In future (2.9?) jackson-databind could then start calling this method for limited case; and users that directly use streaming API may just call this method as necessary.