Add get_delta_tables helper function

[jorritsandbrink]

Description

• adds get_delta_tables helper function
• adds two public methods on Pipeline class (needed the functionality for get_delta_tables and figured they're probably more broadly applicable):
  • get_last_completed_load_id
  • list_loaded_tables_names
• adds new class DeltaReferenceFollowupJob(ReferenceFollowupJob)
  • changes the file extension for Delta reference jobs to reference_delta to distinguish them from "regular reference jobs"
• does some other small unrelated improvements

Related Issues

• Closes Helper that returns delta tables created by pipeline as DeltaTable objects #1651

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[sh-rp]

LGTM

[rudolfix]

LGTM!
note to myself: we should drop circular dependencies ie. Pipeline is imported in destinations which should be level-up in type hierarchy.