Add JSON, SRT, and WebVTT ASR exports

[jmayank1511]

Summary

• add shared structured JSON, SRT, and WebVTT export support
• support both riva_asr_client and riva_streaming_asr_client, with extension inference or explicit --output_format
• automatically request word timestamps for subtitle output, preserve legacy JSON text, and report invalid or failed exports
• keep file export independent from console transcript printing

Testing

• bazel test --workspace_status_command=/bin/true //riva/clients/asr:transcript_exporter_test
• built //riva/clients/asr:riva_asr_client and //riva/clients/asr:riva_streaming_asr_client
• live 30-second and 5-minute JSON/SRT/VTT matrices passed against public Parakeet TDT offline and Parakeet CTC streaming NIMs
• validated JSON structure, subtitle syntax, cue ordering, word timestamps, and cross-format transcript consistency