Add method to query current filename from DataLogBackgroundWriter

[TheTripleV]

Is your feature request related to a problem? Please describe.
I want to live sync data on a coproc with rio wpilog data and want to name it on the coproc the same name as the current wpilog filename.

Describe the solution you'd like
I'd like DataLogBackgroundWriter to have a getter for WriterThreadState::filename

  /**
   * Get current log filename
   */
  std::string_view GetFilename();

  /**
   * Get current log filename
   */
  public String setFilename() {
  }