Add 'JsonGenerator.writeString(Reader r, int charLength)' #17
Description
Current 'writeString()' method requires text value to be passed in its entirety. This may not work well with larger payloads; so we should an alternative method where one can define source for text, used for incremental/streaming read+write.
In addition to Reader, it should be possible to indicate exact number of bytes to include; this because some formats (not JSON however!) require such information to be written before data, or used for initializing write. It should be possible to pass marker value -1 to indicate "not known": not all generators are required to support it, however.
If length is indicated, it is an error for Reader to provide less data (more is ok; generator just will not read anything beyond indicated amount).