Feature request: Specify grids by absolute path

I know we discussed this already but I'm creating this issue for future reference in case we want to implement it in the future.

It would be nice to be able to specify grids with absolute paths, e.g. `dora grid grids/my_grid.py`, in addition to grid names.

The advantages that I see are:
* You can place your grids anywhere you want and run grids from everywhere
* More intuitive in my opinion
* Benefits for free from shell path autocompletion which is super helpful once you start having many grids!