Preliminary DocumentMigrator cleanup

[pgayvallet]

Related to #150301

This PR tend to do some preliminary cleanup before diving into the actual implementation. I'm mostly extracting things from the (way too big and hard to read) document migrator, to increase readability, maintainability and testability (note: I did not add additional unit test for the extracted files, as I did not change the impl and as we'll be rewriting them all soon enough - it would just have been a loss of time)

• Move the document migrator from packages/core/saved-objects/core-saved-objects-migration-server-internal/src/core to it's dedicated folder document_migrator folder
• Extract all the bits not directly performing the document transformation (e.g building the migration map, validation, utils) to their dedicated files
• Only moving things around. No concrete code (apart from types) was modified, to ease managing conflicts with [Saved Objects] Update document migrator not to bump core version on every migration #150443

[kibana-ci]

💚 Build Succeeded

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• Commit: 6d3a72d

Metrics [docs]

✅ unchanged

History

• 💔 Build #106351 failed 72f58fd
• 💔 Build #106350 failed 09624c4

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[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[dokmic]

LGTM 👍

[kibanamachine]

💔 All backports failed

Status	Branch	Result
❌	8.8	The branch "8.8" does not exist

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[kibanamachine]

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You can also create backports manually by running node scripts/backport --pr 150529 locally