[PostHog] Add rate limiting and cooldowns for workflow notifications

### Feature request ### Summary Provide built-in rate limiting and cooldown controls for workflows to prevent sending too many notifications (e.g. emails) to the same user within a given timeframe. --- ### Problem When using cohorts and workflows for targeted notifications: - Users can qualify for multiple cohorts at the same time - This can trigger multiple workflows simultaneously - Leads to excessive notifications (e.g. multiple emails at once) - No straightforward way to enforce limits like "max 3 emails per week" or "48 hours between messages" --- ### Proposed Solution Introduce native rate limiting and cooldown controls: - Allow setting **per-user limits**, such as: - Max X notifications per time window (e.g. 3 per week) - Minimum delay between notifications (e.g. 48-hour cooldown) - Support **global limits across workflows**, not just within a single workflow - Provide configuration at: - Workflow level - Project/global level - Optionally allow prioritization (e.g