api: add dvc.api.exp_show

[dberenbaum]

dvc.api.exp_show should return tabular data similar to dvc exp show (we could use dvc exp show --csv as an example for what the output should look like). It should be possible to get a dataframe from this output with pd.DataFrame(dvc.api.exp_show()).

[dberenbaum]

The args for this method should include the revision selectors args in dvc exp show. Other args from dvc exp show (format types, filtering, sorting, etc.) aren't needed.

[daavoo]

I think it's probably worth waiting for #9170 , but I estimate the implementation as 1 day or less of work