fix(safe_array_union): allow for sorting nested structs

[project-defiant]

✨ Context

This PR closes opentargets/issues#3551

🛠 What does this PR implement

• Addition of fields_order parameter to the safe_array_union function, so we can deal with cses when array contains unordered structs.
• Implement fix for the vep parser when dealing with merge between two variantIndex objects merge of nested array fields.
• Additional helper functions for resolving the column name from pyspark.sql.Column object and sorting structs within the array by fields_order.

🙈 Missing

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[DSuveges]

Crazy. Thanks for cleaning my mess. :D

[ireneisdoomed]

Does this do the same as order_array_of_structs_by_field and order_array_of_structs_by_two_fields?