Notify Slack on auto-deploy branch creation

e4ce7781 - Add block support to the base Slack webhook

Blocks are now Slack's preferred method of message formatting. See https://api.slack.com/reference/messaging/blocks for more information.

dfcb2c05 - Store results of auto-deploy branch creation

The API response from creating a branch is lacking some information, such as what project the branch was created in. The Result struct contains the project, the branch name, and the API response.

227e2de9 - Notify Slack on auto-deploy branch creation

This will post a notification to the Slack incoming webhook defined by AUTO_DEPLOY_NOTIFICATION_URL with the name of the branch and the projects in which it was created.

Example

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Closes https://gitlab.com/gitlab-org/release/framework/issues/360