Add SFTP publisher service

Closes #106 (closed)

Adds an SFTP publisher to opmet backend.

To test:

Backend:

• Run the backend using docker-compose-sftp.yml. It adds an SFTP server to the development compose file.
  • Run docker compose -f docker-compose-sftp.yml up

Frontend:

• Configure to use your local opmet backend instance
• nx serve
• Open the sigmet/airmet preset
• Publish a sigmet/airmet

The published files will appear in sftp_output subdir in your opmet backend directory.