plots/metrics/params: allow comparing between different paths

[pared]

In some use cases
user might want to compare different metrics in different paths.

We need to discuss if and how we are going to support it.
My initial idea is to expand how we provide rev :
instead of dvc plots diff tag1 we could go with dvc plots diff tag1:{path}

[qthurier]

This would be very helpful for example when experiments are isolated in different folders on the same branch (as opposed to each experiment being on a dedicated branch).

The scenario comparing metric1.json from branch1 with metric2.json from branch2 can be helpful too but it is not my primary use case. I think ideally we should be able to perform diff on 2 combination of (path, revision).

[pared]

Closing in favour of #5693