Implement the client credentials grant flow for the Oauth 2.0 integration implemented for Incoming/Outgoing Mail Server

Issue Summary

The Oauth 2.0 authentication for the Outgoing Mail Server functionality which was requested in this feature request was introduced in Jira 9.2.0 as per Jira 9.2 release notes.

Currently, the Oauth 2.0 integration implemented in Jira only supports OAuth 2.0 auth code grant flow, which means that, to configure the Outgoing Mail Server with Oauth 2.0, it is required to authorize the Jira application by logging MS with a Microsoft User. As a result, the Jira application cannot authorize the SMTP mailbox itself.

Note

There is the same limitation for the Jira Incoming mail Server configuration: the Oauth 2.0 integration that was implemented for Jira's Mail Server also only supports the OAuth 2.0 auth code grant flow

Suggestion

Implement a server-to-server interaction, so that no "human user" has to manually log into Microsoft to authorize the integration, which is basically how the client credentials grant flow works.

This implementation should work with both:

• the Jira outgoing mail server configuration
• the Jira incoming mail server configuration