feat(medusa): Nested Categories Admin List Endpoint (#2973)

* chore: added get route for admin categories API

* chore: add tree method to mock repository

* chore: added changeset to the PR

* chore: rename id to productCategoryId in service

* chore: switch cli option to string

* chore: lint fixes, tests for parent category

* chore: move Nested Categories behind feature flag

* chore: use transformQuery hook in api

* chore: add feature flag in migrations

* chore: remove migration FF, fix FF name

* chore: add free text search + count repo function

* chore: added list endpoint for admin

* chore: added changeset for feature

* chore: address pr review comments

* chore: change oas comment

* chore: add nullable parent category filter + test

.changeset/silent-spiders-battle.md

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+---
+"@medusajs/medusa": patch
+---
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+feat(nested-categories) adds a list endpoint to admin nested categories