Support tab & special delimiter chars in csv2avro converter

The original ConvertCSVToAvro implementation was unable to process tab separated files, because "\t" as delimiter character was invalid, coming from the UI, entered by the user. The validator for this property only allowed single characters and the "\t" input was not unescaped before validation. The current version allows special, escaped characters like \t and \f (form-feed) for delimiter, quote and escape character.