Mark BulkImports::RelationExportWorker as sidekiq memory bound

What does this MR do and why?

This MR marks BulkImports::RelationExportWorker as memory bound, in order to move execution of these jobs to memory-bound sidekiq shard, which is not auto-scaled on .com & has limited resources It eliminates job interruptions, if relation that is exported has a lot of data and takes a long time to export. Examples of job interruption can be found here.

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