Textarea Field Type for Rule Configuration

Rule actions now support a textarea field type for multiline input. This powers Slack's message template editor and is available for any integration that needs multiline configuration in rule setup.

Previously, rule action fields only supported single-line text and password inputs. The new textarea type renders a resizable monospace text area in the rule configuration form - useful for templates, message bodies, or any structured text input.

This is a platform-level change, not specific to Slack. Any integration can now define a kind: "field" action data entry with type: "textarea" and get multiline input in the rule form automatically.