Make Identity Pool / IAM authorization optional, fall back to JWT authorization if not provided

[shaunhurley]

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Hiya, another unsolicited pull request, sorry! Super useful test utility, but I'm doing a bunch of JWT authorization based stuff.

Originally went with a separate makeJWTRequest() function and bypassed getCredentials altogether approach, however figured this approach was less duplication.

Only other thought was potentially pulling out additionalParams to a global, so I could handle the Authorization header in the getCredentials() bypass and avoid doing another argv.identityPoolId check in makeRequest(), which would make things a little cleaner, but didn't implement. Not sure on your philosophy on globals :)