Support JWT as an authentication method

[cdmadrigal]

BUSINESS PROBLEM
Today the integration does not support JWT as an authentication method for the plugin. Adding JWT support would bring the integration to parity with other Venafi integrations. VCert SDK and VCert CLI already support JWT, so it's a matter of making those variables available to be passed from the plugin. Today OAuth is the primary authentication method within the plugin.

PROPOSED SOLUTION
Allow JWT an an authentication method.

CURRENT ALTERNATIVES
None. Currently authenticate with Oauth.