This PR simplifies state in test webhooks so that it can be cached
easily. Caching this state will allow us to start using Redis for manual
webhooks, to support manual webhooks to work in multi-main setup.
- [x] Convert `workflowWebhooks` to a getter - no need to optimize for
deactivation
- [x] Remove array from value in `TestWebhooks.webhookUrls`
- [x] Consolidate `webhookUrls` and `registeredWebhooks`
Remove duplication, improve readability, and expand tests for
`TestWebhooks.ts` - in anticipation for storing test webhooks in Redis.
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Co-authored-by: कारतोफ्फेलस्क्रिप्ट™ <aditya@netroy.in>