Add customized Scalars charts capabilities

[lions1988]

Enhance Scalars graphs to add user-customization capabilities
In particular, we are looking for a way to aggregate chosen metrics (upon user request in the UI wizard) into single graph

For example, examine loss metric, in eval and train phases on single chart
At the moment, we can see the metric for each phase on separate chart, e.g:

Since each task might have different metrics that task owner wants to focus, please consider developing this feature, so it will be possible to customize the charts upon user request and provide the ability to generate "charts on-demand", based on user request and the task data

[ainoam]

Thanks for suggesting @lions1988 - we'll consider how this can best fit into ClearML's future roadmap.