[CsvIO]: Create CsvIO::mapContextualTextIORowToKV

[francisohara24]

What needs to happen?

Create a helper method within CsvIO for processing the PCollection<Row> output of ContextualTextIO.ReadFiles into a PCollection<KV<String, String>>.

• CsvIO.ReadFiles, the main transform for reading CSV files within CsvIO, will rely on ContextualTextIO.ReadFiles for parsing CSV files into CSV record strings for further processing.
• A helper method is therefore required for converting the output PCollection of ContextualTextIO.ReadFiles into a PCollection of KV mapping each CSV record (String) to its file name (String).

Issue Priority

Priority: 2 (default / most normal work should be filed as P2)

Issue Components

• Component: Python SDK
• Component: Java SDK
• Component: Go SDK
• Component: Typescript SDK
• Component: IO connector
• Component: Beam YAML
• Component: Beam examples
• Component: Beam playground
• Component: Beam katas
• Component: Website
• Component: Spark Runner
• Component: Flink Runner
• Component: Samza Runner
• Component: Twister2 Runner
• Component: Hazelcast Jet Runner
• Component: Google Cloud Dataflow Runner