exp: random sweeps

[dberenbaum]

As a follow up to #8083, it would be helpful to have support for random search. It's not possible with the basic sweeper in hydra.

Possible solutions:

1. Add support for random search override syntax in the hydra basic sweeper (https://hydra.cc/docs/advanced/override_grammar/extended/#sweeps). Since random search needs to select random combinations across multiple parameters, I think this would require another top-level parameter to designate a random sweep and additional parameters (most importantly, the number of experiments to randomly try).
2. Add a new hydra sweeper for random search.
3. Add DVC-specific syntax for random search.

cc @alex000kim