Today when adding a json parameter in queries, the serialized json document in bytes gets copied multiple times. One time in JsonHandler.SerializeJsonDocument when writing the byte array, which is then copied once the function returns. If this function gets called frequently with large documents, blocks will become allocated on LOH which is far from ideal in high performance critical application.

This change allows me to use strategies such as buffer pooling when building the JSON object, for example:

var document = JsonSerializer.SerializeToDocument(obj);
var documentBytes = SerializeToBytes(document);

... 

new NpgsqlParameter("document", NpgsqlDbType.Jsonb)
{
    Value = document,
    ConvertedValue = documentBytes
}

The function SerializeToBytes can then use for example a RecyclableMemoryStream to re-use the memory being used for sending the documents.

Related to the discussion here: #1727