Update BulkImports::Pipeline to make use of DSL methods optional

What does this MR do?

This MR:

• Update BulkImports::Pipeline to make use of DSL methods optional. Sometimes extraction/transformation/loading is fairly small so there is no need to define a whole class for 1 line of code. Instead, make it optional so that you can define extract/transform/load method within the pipeline itself
• Removes the required initialisation params for pipeline parts (extractors, transformers and loader) so that there is no need to define initialize method when it's not required
• Removes some of the 1-liner loaders and puts loading within respective pipelines

Mentions #300774 (closed)

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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

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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