Add use_clean_redis_caching around hook for specs

What does this MR do?

This adds a new rspec hook, use_clean_redis_caching, which when used allows Rails.cache to write to Redis. This is similar to clean_gitlab_redis_cache, which is used when Redis is interfaced with directly.

Importantly, this sets the namespace which mimicks the setup we use in application.rb. This is important because it allows us to test code which uses redis directly to read/modify redis keys saved via Rails.cache.write.

An example of this is here.

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