Feature: add optional rng argument to `Base.rand` methods

[apkille]

This will be valuable for tests that pass stable random numbers generated from StableRNGs.jl.

[alfranmu]

Hey! I was actually just looking for this feature and saw this issue. I'm new to contributing but eager to learn. Is anyone currently working on it? I'd love to give it a try.

[apkille]

Go right ahead! I can help review PRs.