Add goal line functionality to funnel historical trends

Feature request

Is your feature request related to a problem?

When analyzing funnel conversion rates over time using historical trends, there's currently no way to visualize target conversion rates as reference points. Goal lines are available for trend insights, but this functionality is missing for funnel historical trends, making it difficult to track if conversion rates are meeting desired targets.

Describe the solution you'd like

Add goal line functionality to funnel historical trends, similar to how it works in trend line insights. This would allow users to:

• Set target conversion rates as horizontal goal lines
• Visually track actual conversion rates against goals
• Monitor KPI targets over time

Describe alternatives you've considered

Currently using regular trend insights with goal lines for basic metrics, but this doesn't provide the rich funnel conversion analysis available in funnel historical trends.

Additional context

This would be particularly valuable for:

• Monitoring funnel performance against KPIs
• Analyzing seasonal variations against baseline goals
• Evaluating the impact of product changes on conversion rates
• Understanding if conversion improvements are meeting targets

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]

No files, tests, or entry points are identified. Start by locating the existing goal-line implementation for trend insights and the funnel historical trends visualization, then determine how the two paths represent and render conversion-rate targets. Done means users can configure horizontal goal lines for funnel historical trends and compare them with actual conversion rates, with coverage for the new behavior.