Limit Group Migration extractors and loaders to 1 per pipeline

What does this MR do?

Currently Bulk Imports Group Migration BulkImports::Pipeline::Runner has an ability to add multiple extractors & loaders.

e.g.

      class MyPipeline
        include Pipeline

        extractor Extractor
        extractor AnotherExtractor

        transformer Transformer 

        loader Loader
        loader AnotherLoader
      end

Because of that, BulkImports::Pipeline::Runner#run has to loop through extractors/loaders, increasing it's complexity. In reality, so far (and it might change in the future), there is no need in having multiple extractors or loaders. Usually it's 1 loader and 1 extractor per pipeline.

This MR restricts defining extractors/loaders to 1 each per pipeline in order to reduce BulkImports::Pipeline::Runner#run complexity. If we are going to need to extract/load from/to multiple sources/destination we can revisit it and consider adding such functionality in the future, but there is no need in having it now.

Mentions #283945 (closed)

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