refactor: Add background Redux slice as proxy for unflattened background state

[MajorLift]

Overview

Instead of unflattening the metamask Redux slice in-place, a new background slice has been created as a placeholder or eventual replacement.

Adopting a "horizontal sharding" approach, we will be iteratively transferring each controller's state properties from metamask to background, simultaneously updating downstream references to point to the background slice.

Pros

• Less disruptive, as existing code still functions like before with the unaffected metamask slice.
• Avoids causing metamask slice structure to be inconsistent and in flux while the transition takes place, introducing difficulties for feature development and potentially causing bugs.
• Avoids triggering the flattening operation on every background state update, along with the potentially significant performance penalties.
• Enables a complete transition of upstream logic for hydrating Redux with background state, and emitting patches for state updates, without needing to temporarily maintain duplicate versions of PatchStore and ComposableObservableStore.

Cons

• Introduces temporary dead code into the codebase.

  • The purpose of the background slice will be documented extensively through comments and internal communications, so that feature teams are aware not to reference or modify it.

• Significantly increased memory usage

  • Filter patches sent to background slice so that only migrated controller state is populated and updated.
    • wrap background slice with devtime feature flag
  • e2e tests ensuring no failure in e.g. low-powered devices affected by memory
    • safe to rely on existing tests and qa? probably not blocker
    • see acceptance criteria no. 2 (no breaking changes, bugs)

• selector disentanglement for independent prs.

  • only need to update "leaf-node" selectors.

Consolidation of test mock objects (x) -> go through them individually to unflatten
Priority/urgency of clean-up tasks in phase 3 (especially regarding realization of selector memoization performance benefits)

Description

Related issues

Fixes:

Manual testing steps

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Screenshots/Recordings

Before

After

Pre-merge author checklist

• I've followed MetaMask Contributor Docs and MetaMask Extension Coding Standards.
• I've completed the PR template to the best of my ability
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable
• I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

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