Support for rotating pipeline webhook URLs

We've added a pipelineRotateWebhookURL GraphQL mutation for rotating the webhook URL that connects a pipeline with your source control. And a github-webhook-rotate CLI tool for automating the process with GitHub 🌪

You can find the CLI tool on GitHub at buildkite/github-webhook-rotate, and the GraphQL mutation documentation by searching for pipelineRotateWebhookURL in the GraphQL Explorer.