Connect experiments table file columns to data

[dberenbaum]

The experiments table shows the hash of dvc-tracked files associated with the experiment in purple like this:

This is useful to quickly identify when files have changed but is limited. We could make it possible to checkout, open, and compare these files from within VS Code. Examples of helpful functionality we could add for files:

1. Checkout that version of the file to the workspace (not the entire experiment, just the file).
2. Open a diff in the VS Code editor against the workspace version (dvc diff or some dataset management tooling dvc#770). Obviously, this is only helpful for non-binary files, but I think it's fine to start generally and let users decide where it's useful.

What about directories? We could:

1. Checkout the directory contents to the workspace.
2. Do a dvc diff against the workspace version.

[dberenbaum]

Related ideas:

• Show the status of these files for the workspace row. For example, we could add a letter next to the hash to indicate the status or color them by status. Users can see this from the SCM view, but it might help to connect that info to the experiments view.
• Show the DAG for every row. We could try to show a hash of the dag contents so it's possible to see when the DAG changed, and it would be nice to be able to open the DAG for any row (and maybe even show a diff of the DAG between two rows).

We could also consider similar features in Studio.