Add Unify Circuit project integration service

What does this MR do?

This merge request adds Unify Circuit as a new project service, which allows submitting GitLab events as notifications to this service. Unify Circuit is a fully web based RTC and collaboration tool, which also offers a free tiers.

It is functionality and implementation wise similar to the Hangout project service, but utilises the Markdown capability of Unify Circuit.

🔧 with ❤ at Siemens

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team