Add a strvals source

[na--]

github.com/kubernetes/helm/pkg/strvals is a very nice config format that allows complex hierarchical configurations to be encoded in a single comma-separated value. Pretty much meant to be a flattened YAML, from what I understand...

We used it in several places in k6, but dropped it for simpler hand-roller parses in the few places where it was used, so we could drop the relatively big dependency (grafana/k6#926). Still, if we can implement it in a sane manner, it will be quite useful. Especially if we can actually combine it with the environment variable and CLI flags source, it would allow for some quite flexible configurations!

[mstoykov]

the current implementation used in loki's config where the more "complex" parts are actually used.