Re-add Slack integration

[jongpie]

In an old beta release of Logger, there was built-in support for sending alerts to Slack for certain logs/log entries - I ended up removing it to focus on the core logging functionality, but it'd be great to have an add-on package that can be installed on top of Logger in orgs that want to integrate with Slack.

Design

• The entire integration should be 100% optional - not every org uses Slack, so no extra metadata for this integration should be included in the nebula-logger directory
  • TODO: need to decide if this will (dynamically?) work for both versions of the package (unlocked & managed w/ namespace), but to start, it'll probably be just for the unlocked package (no namespace)
• There should be a way to control which logs/log entries are pushed to Slack - the old integration used a checkbox field on the records, that may still be the best way to handle it
• There should be a declarative way to control the message string sent to Slack - the base message could be stored as a custom label or in a CMDT object
• There should be a way to declaratively set/update the Slack details (API key, channel, etc.)