Add Pumble integration

What does this MR do and why?

This merge request adds Pumble as a new project integration, which allows submitting GitLab events as notifications to this service.

It is functionality and implementation-wise similar to the Webex Teams project service.

Screenshots or screen recordings

Pumble webhook configuration:

GitLab integration configuration:

Pumble client:

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.