How To authenticate the third party application from Jira Cloud App

Hi Team,

I am developing the Jira app for the third party application.

And I am facing the difficulties while developing the authentication code for the third party application, However, I can find the documents for the Authenticate Atlassian Connect App but I couldn’t get any clear cut idea on how could develop the authentication for third-party application from Jira cloud app.

For eg: how to authenticate google contacts from Jira cloud app and list all the google contacts in Jira issue to assign google contacts person to handle the Jira issue.

Could you please assist me with the demo or detailed sample code for better understanding.