Avatar upload support for Group Create/Update API

What does this MR do?

Fixes #36681 (closed)

The group model already had support to store avatar. We did not have API support to upload avatars while creating/updating groups. This MR adds the param to group params. There was no change in groups/create_service.rb or groups/update_service.rb.

Last, this MR has specs for the new params and a docstring to let users know that the API now supports uploading of avatar while creating and updating groups.

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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