Cleanup env vars beyond `ENV_API_KEY_NAME`.

[daavoo]

Some providers are using cusotm env vars (that are not really documented in the provider sdk) to get some config values (i.e. api_base).

I think we should not fetch env vars under the hood (beyond the documented ENV_API_KEY_NAME) and instead let the underlying sdk handle it and/or document how users can do it themselves at their code level via os.getenv