compare-perf: Add support for selecting a single run as input

[tausbn]

A very hacky implementation that simply instantiates an empty PerformanceOverviewScanner as the "from" column (i.e. with all values empty). To see it in action, select a single query run in the query history and pick "Compare Performance" from the context menu. Then select the "Single run" option when prompted.

Second commit takes care of simplifying the view when not in comparison mode.

[asgerf]

Looks great! 🚢

[esbena]

works for me locally with hack to support external logs as well, that should be trial-by-fire enough

[esbena]

Let's merge this before https://github.com/github/vscode-codeql/pull/3819 - I know the semantic conflicts that have to be addressed.

[esbena]

I'll immediately do some cleanups to this once it is merged. I have the commits ready locally..