GlBarChart: Add scoped slots for tooltip customization

What does this MR do?

Introduces tooltip-title, tooltip-content and tooltip-value scoped slots to allow consumers to customize the chart's tooltip.

Why?

Analytics dashboards currently uses the BarChart visualization – which wraps GlBarChart – but lacks tooltip customization options. This limitation prevents teams from tailoring tooltip content to their specific analytics needs within the dashboard framework. Adding these scoped slots to the underlying GlBarChart component will enable BarChart visualizations in the framework to support custom tooltip rendering while maintaining consistency with our other chart components.

Screenshots or screen recordings

Default	Custom tooltip	Custom tooltip value

Integrations

• GitLab: gitlab-org/gitlab!204249 (closed)

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