[Feature Request] Accept slack webhook url in addition to bot token/channel id

Supporting slack webhook url would be helpful. Creating a bot within a large org is sometimes a lot harder than getting a webhook url

No files or tests are named in the issue. Start by locating the existing Slack bot token and channel ID configuration and notification entry point; done means a Slack webhook URL can be configured and used to send the same report notifications without requiring a bot.

Written by the indexing model from the issue text.