Allow subclasses of BasePipeline to resolved

What does this MR do?

Currently, one can only pass Symbols to Pipeline.[] which is convenient in most cases assuming that the pipeline is namespaced to Banzai::Pipeline.

In some cases, we want a namespace a pipeline to a specific domain namespace (e.g StatusPage::PostProcessPipeline). In those cases we had to pass an underscored version of the namespaced class name (e.g. :'_status_page::_post_process_pipeline' or :'status_page/post_process_pipeline').

This commit allows passing a subclass of Banzai::Pipeline::BasePipeline to Pipeline.[].

This came up during a review of !28052 (comment 323438355).

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