Add ability to authenticate to conjur via different authenticators

[AndrewCopeland]

Add the ability to the cli and api client to support a flag such as:

conjur-cli --insecure -l https://myserver -a orgname -u "host/aws/<aws-account-id>/<iam-role-name>" --authn iam --authn-id authServiceId \
  variable get foo/bar

Where the --authn flag should support both iam and k8s and when this flag is provided a -p should not be required. If the --authn is provided then the --authn-id will also be required since this is used to authenticate correctly.

The main difference between the authenticators is just the 'authenticate' endpoint being used, the rest of the cli/api client should remain unchanged.