Introduce Pipeline abstract controller

Part of #220605 (closed) & #211838 (closed)

What does this MR do?

This MR introduces a new Abstract controller which will be used to inherit new controllers performing actions in a pipeline context.

This was brought up during a conversation with @ayufan in #220605 (comment 356393226)

If our intent is to have a controller that we can inherit from, this could be Projects::(Base|Abstract)PipelinesController where the PipelinesController/Stages/TestsControllers would inherit from.

This is a similar pattern we are using across our codebase, see:

• Groups
• Boards

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

cc @ayufan