feat: Add feature flag to treat local Snaps as preinstalled

[Mrtenz]

This adds a new feature flag forcePreinstalledSnaps, which makes the SnapController treat any local Snaps as preinstalled Snaps.

It's intended to only be enabled in local Flask builds, and should not be enabled in any production builds. It can be used to simplify the development of preinstalled Snaps by using a local Snap instead.

Related extension PR: MetaMask/metamask-extension#34022.

Closes #3517.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.25%. Comparing base (46c9eb9) to head (136c886).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3523      +/-   ##
==========================================
+ Coverage   98.24%   98.25%   +0.01%     
==========================================
  Files         408      410       +2     
  Lines       11520    11588      +68     
  Branches     1795     1804       +9     
==========================================
+ Hits        11318    11386      +68     
  Misses        202      202              

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