feat: Support multiple source buckets for data migration

[antroy-madetech]

Move to a nested structure to support multiple source buckets for data migration

Addresses DBTP-1635

Docs PR: https://github.com/uktrade/platform-documentation/pull/457

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Checklist:

Title:

• Scope included as per conventional commits
• Ticket reference included (unless it's a quick out of ticket thing)

Description:

• Link to ticket included (unless it's a quick out of ticket thing)
• Includes tests (or an explanation for why it doesn't)
• Includes any applicable changes to the documentation in this code base
• Includes link(s) to any applicable changes to the documentation in the DBT Platform Documentation (can be to a pull request)

Tasks:

• Trigger the pull request regression tests for this branch and confirm that they are passing

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.83%. Comparing base (155f371) to head (92d259f).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #321   +/-   ##
=======================================
  Coverage   80.83%   80.83%           
=======================================
  Files           5        5           
  Lines         814      814           
=======================================
  Hits          658      658           
  Misses        156      156           

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[ejayesh]

lgtm

[WillGibson]

Nice. Couple of small suggestions, but approving.

[WillGibson]

Probably worth including a comment above import mentioning that it is deprecated.