add slack reports

[nurbal]

Adds slack reports.

When a config.logging.slack section is present, it sends a report with error messages to the specified slack channel.
All you have to do is to provide slack credentials in the config file:

  logging:
    log_level: INFO
    OTLP_endpoint: http://loki01.server.mila.quebec:3100/otlp/v1/logs
    service_name: sarc-dev
    slack: 
      description: SARC-test
      token: <your-slack-app-token-here>
      channel: <your-channel-ID-here>