Support providing api credentials from environment

[elthariel]

Is your feature request related to a problem? Please describe.

When using the API in CI environments, it's inconvenient and sometimes impossible to rely on ~/config/runpod.toml. It's a common practice in those cases to supply those credentials using an environment variables

Describe the solution you'd like
I'd like to see this in __init__py:

_credentials = get_credentials(profile)
_env_api_key = os.environ.get('RUNPOD_API_KEY', None)
if _env_api_key is not None:
     api_key = _env_api_key
elif _credentials is not None:
    api_key = _credentials["api_key"]  # pylint: disable=invalid-name
else:
    api_key = None  # pylint: disable=invalid-name

Describe alternatives you've considered

My CI jobs are currently writing into the config file but this is extremely annoying and prone to race conditions