Simplify BulkImports::Pipeline::Context

What does this MR do?

As the Gitlab Migration Grows (BulkImports) more and more developers have to access the Pipeline Context data, like the entity's group. To make this easier, the Context now has syntax-sugar methods for the most accessed data.

Related to: #300719 (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