[SOAR-20441] Zendesk to support other authentication methods

[lcwiklinski-r7]

https://rapid7.atlassian.net/browse/SOAR-20441

• I've removed username and password authentication type, because it won't be supported from the 12th of January https://support.zendesk.com/hc/en-us/articles/8843683464346-Important-update-Password-access-for-APIs-will-soon-be-removed
• I've replaced it with API Token-based authentication only.
• Fixed unit tests

[simonirwin-r7]

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[igorski-r7]

Please update base from master to release branch