Allow for configurable auth

[DarthHater]

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• What are you trying to do?

You should be able to specify an API key to use for cargo-pants, so that someone can use their own key.

As well, you can send a basic auth header of email:password, instead of the token, so we'd want to handle that path too.

• What feature or behavior is this required for?

This allows someone to essentially auth with OSSIndex.

• How could we solve this issue? (Not knowing is okay!)

Probably pick up an API Key from an environment variable, or something similar. For the email:password combo, build basic auth capabilities and probably pick this up from a file, maybe:

~/.cargo-pants/creds.something

• Anything else?

Should also allow for reading credentials from environment variables.
For consistency, should follow the same naming as nancy, eg:

export [email protected]
export OSSI_TOKEN=A4@k3@p1T0k3n

and for IQ:

export [email protected]
export OSSI_TOKEN=A4@k3@p1T0k3n
export IQ_USERNAME=nondefaultuser
export IQ_TOKEN=yourtoken
export IQ_SERVER=http://adifferentserverurl:port