OpenSystemsDevelopment / qpdf

For now, we are using codecov as an internal tool and not advertising
code coverage. I scrutinize coverage on include and libqpdf but don't
want to remove coverage reporting for other areas.

Add test for QPDFObjectHandle::isDirectNull

Add test for QPDFObjectHandle::getStreamJSON

As of this moment, if a specific commit's coverage data is uploaded to
CodeCov via a pull request, when that exact same commit builds in
main, the branch is not associated with the commit. I'm hoping this
might work around that.

Still need to handle upload artifacts, but that requires additional
changes.

Add additional sparse QPDF_Array tests

Use QPDF_Stream::writeStreamJSON.
Factor out the json_stream_data == qpdf_sj_file case.

(Replacing the temporary implementation from the last commit.)

writeStreamJSON is a temporary implementation minimally adapted from
getStreamJSON.

Also, test writing JSON v1 files and files with deeply nested containers.

Provide a custom method to check whether a name is valid utf8. Integrate
checking for characters that need to be escaped in JSON.

Create an alternative to getJSON to allow an object handle to be written as JSON without the overhead of creating a JSON object.

Create a simple utility class for writing JSON to a pipeline.