Add support for build failed/succeeded notifications to the `oci-build` workflow

[enpaul]

Add support for build success/failure notifications to be sent to slack using slackapi/slack-github-action

Add two new optional inputs:

slack-on-failure: bool
slack-on-success: bool

And a new secret:

slack-token: string

[soleilera]

Since this is the last piece of work Ethan has laid out for the Codefresh migration, I want to nominate it as a quarterly candidate.