ci: wrapper for step level custom slack notifications

Created by: jhchabran

As we were talking with @Strum355 about turning the code intel QA tests into a soft fail, but still getting the notifications, I went ahead and wrappeg the plugin we're using in preprod https://github.com/sourcegraph/step-slack-notify-buildkite-plugin to make it easy to use.

Test plan

Tests, see https://github.com/sourcegraph/sourcegraph/commit/430c4c4c8d6ce403ee6a974351b3fac7ed5bfae3#diff-d6bc40d707edf58af7324d8f5b9e6836637e1d768dcae12d6338a4b324c0a273R882 and the results https://buildkite.com/sourcegraph/sourcegraph/builds/159935#0181f287-feae-48c8-a1ce-d726f3acb274