Optional custom icon in the OmniAuth login labels

What does this MR do?

Implements #17437 (closed) - adds an optional 'icon' attribute to the Omniauth provider configuration that can be used to make the custom login providers stand out more. Credits go to the developer of the original MR https://gitlab.com/geomunion @geomunion - I have only rebased that to the current sources and added the necessary documentation. Unfortunately I'm not a Ruby dev and I can't really help with tests.

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
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• 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• 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
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