Add string as an option for decimal.handling.mode

Currently, debezium provides a configuration paramter for numeric data types which can be either precise or double. If we choose precise, the data is converted to byte array by kafka and is seen in the kafka topic as base 64 string (for JSON messages). This is very difficult to convert back to numbers. Since many sink databases support numbers within double quoted string, it would be useful if the number was returned as a string so that no conversion is required.
This option should simply return the number value as a string like

 decimal.toPlainString()