Add elementary slack webhook for data quality alerts

[chowbao]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with the jira ticket associated with the PR.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated the README with the added features, breaking changes, new instructions on how to use the repository.

What

Add elementary slack web hook for data quality alerts

Why

Slack elementary app oauth token isn't working correctly. Using the elementary slack web hook option as a temporary solution to get data quality alerts back

Known limitations

Exposes public web hook instead of using oauth tokens

[sydneynotthecity]

This PR can be closed as we set up Slack auth via token