#666 Add support for record length value mapping

[yruslan]

Example:

val df = spark.read
  .format("cobol")
  .option("copybook_contents", copybook)
  .option("record_format", "F")
  .option("record_length_field", "SEG-ID")
  .option("record_length_map", """{"A":4,"B":7,"C":8}""") // <---- this
  .load(tempFile)

[github-actions]

JaCoCo code coverage report - 'cobol-parser'

File	Coverage [64.74%]	🍏
RecordLengthField.scala	100%	🍏
ReaderParametersValidator.scala	73.33%	🍏
VarLenNestedReader.scala	71.5%	🍏
VRLRecordReader.scala	56.72%	🍏

Total Project Coverage	85.58%	🍏

[github-actions]

JaCoCo code coverage report - 'spark-cobol'

File	Coverage [84.75%]	🍏
CobolParametersParser.scala	84.94%	🍏
CobolScanners.scala	82.56%	🍏

Total Project Coverage	80.22%	🍏