Use some lightweight secret management thing

[dlwh]

I'm tired of passing env variables around. should use some not-too-complicated secrets thing. If it's just as easy, I'd prefer something cloud agnostic, but gcp secrets is probably fine. Ideally levanter's python code would know nothing about it though and just use env variables or an env file (which I know isn't best practice, but I'm not actually worried about my HF or WANDB key)

[dlwh]

fixed in #601 (🙇 @rjpower )