fix(slack): make slack work with distributed tests

[InesNi]

Description

1. Enabling Slack Plugin for Fargate runs:

• Manually instantiating the plugin in run-cluster.js similar to ensure plugin. ( plugins are not automatically loaded in main thread for Fargate runs)
• Disabling slack in cloud workers to avoid sending duplicate notifications.
• Setting the global.artillery.cloudEnabled variable in run-fargate. slack uses this variable to check if a run URL needs to be included in the notification. It is set in run.js when cloud is enabled but was missing in run-fargate
• Adding slack to both official plugin lists

2. Enabling Slack Plugin for Lambda runs

• Setting the WORKER_ID to process.env
• Works with containerized Lambda (tested manually) and should work with .zip file version but can't test easily as local changes from other unreleased packages aren't reflected

3. A few additional smaller changes have been added:

• As a tweak to duration - seconds are ceiled rather than floored so that duration in notification doesn't always appear to be 1 second less that the duration set in the test script.
• A few more debug messages were added to make debugging easier

Testing

• Manually ran multiple tests to ensure it works as expected:
  • with varied arrivalRate, duration and container count
  • with and without ensure plugin as well as with and without failed checks.
  • with and without errors
  • for tests that are recorded to Artillery Cloud and ones that are not.

Pre-merge checklist

• Does this require an update to the docs?
• Does this require a changelog entry?