## Explanation

This PR moves the logic related to account syncing from
`UserStorageController` to separated files in the `account-syncing`
folder.
It also improves test coverage related to account syncing to 100%.


## References

Related to #4923

- Extension draft PR:
MetaMask/metamask-extension#29316
  - CI & E2E Passes ✅ 
- I needed to add the new `isAccountSyncingInProgress` state key at
various places to make it pass CI (as expected)
  - Account syncing is enabled on extension
- Mobile draft PR:
MetaMask/metamask-mobile#12755
  - CI passes BUT ✅ ☝️  
- I needed to add the new isAccountSyncingInProgress state key at
various places to make it pass CI (as expected)
- UTs were broken. It seems to be linked to the latest version of
`NetworkController` and its `NetworkController:networkDidChange` event.
This latest version is requested by our controller as part of the
upcoming network syncing feature.
- This will require a separate PR that bumps `NetworkController` to
`v22.1.1` (or another incriminated dependency TBD)
- This has started to be addressed here:
MetaMask/metamask-mobile#12765
- This was fixed by this PR: #5116
  - In any case, account syncing is **NOT** enabled on mobile yet

## Changelog

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### `@metamask/profile-sync-controller`

- **CHANGED**: moved account syncing logic to its own files
- **BREAKING**: added a new `isAccountSyncingInProgress` state key

## Checklist

- [x] I've updated the test suite for new or updated code as appropriate
- [x] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [x] I've highlighted breaking changes using the "BREAKING" category
above as appropriate
- [x] I've prepared draft pull requests for clients and consumer
packages to resolve any breaking changes