Namespace CI Pipeline Mutations

What does this MR do?

This MR namespaces the existing Ci mutations under Ci::Pipeline

Why ?

I went to add a mutation to CI mutations for the ProjectCiCdSettings model and I realized that the Ci::Base class did not apply to my mutation since it was not a mutation that takes a pipeline id. I decided that it would make sense to namespace this code under Ci::Pipeline given that the Base mutation only applies to mutations that take a PipelineID as an argument. This pattern is used in other places in the codebase notably boards/lists.

Screenshots (strongly suggested)

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