Add dry run option

Adds a --dry-run CLI argument that when passed skips the update_issue_with_note and send_slack_message.

The script will still find issues and classify them but it will only report the classification to stdout.

The classify job has DRY_RUN set to true in MRs so that the job can be run as a dry run by default.

Example output:

https://gitlab.com/gitlab-org/ml-ops/tanuki-stan/-/jobs/4055019233

Performing a dry run. Issues will not be updated.
https://gitlab.com/api/v4/projects/278964/issues?state=opened&not[labels]=automation%3Aml%2CEngineering+Productivity%2Cgroup%3A%3A%2A&per_page=100&created_after=2023-04-03T00:00:00&page=1
:red_circle: <https://gitlab.com/gitlab-org/gitlab/issues/404404|Issue #404404: Design the ClickHouse schema for `audit_events`>: did not apply label `group::threat insights` (probability 0.08)