Key is a required field for the Slack Trigger, but not in examples

### Describe the issue

I'm trying to use the Slack trigger and I noticed my flow code editor threw a warning for not having a required field - key. This should be added to the examples.

> You can use a random key generator to create the key.

🤔 Is there a way we can do random key generation directly in Kestra flow code? Something like `"{{ randomInt(32) }}"`? Or is there a better recommendation/pattern?

### Environment

- Kestra Version: develop

Start by locating the Slack Trigger configuration examples and identify where the required key is omitted. Update the examples to include the key, then verify that the recommended approach for generating or supplying it is clear; no specific test or file is named in the issue.

Written by the indexing model from the issue text.