Add Tracing Details Span Chart

What does this MR do and why?

• Add gantt-like chart to tracing details to represent trace's spans. (It mimics the Roadmap gantt chart)
• Services are assigned colours based on the https://design.gitlab.com/data-visualization/color#categorical-data
• Removing temporary changes where trace duration was calculated on the client
• Update mocks

Other resources:

• Figma prototype
• Previous Discussion: !128558 (comment 1503335863)
• Closes frontend: Trace Details UI (gitlab-org/opstrace/opstrace#2253 - closed)

Screenshots or screen recordings

How to set up and validate locally

• Enable FF Feature.enable(:observability_tracing)
• Apply patch to load local mocks ( as running the whole stack is not trivial )

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.