Support utf8 encoding for snap_getFile

[FrederikBolding]

Support utf8 encoding for snap_getFile. This is useful for loading JSON or other text files.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (cffc3c3) 96.03% compared to head (77f17a4) 96.06%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1858      +/-   ##
==========================================
+ Coverage   96.03%   96.06%   +0.02%     
==========================================
  Files         241      243       +2     
  Lines        5573     5663      +90     
  Branches      877      895      +18     
==========================================
+ Hits         5352     5440      +88     
- Misses        221      223       +2     

Files	Coverage Δ
packages/rpc-methods/src/permitted/getFile.ts	100.00% <100.00%> (ø)
packages/rpc-methods/src/permitted/handlers.ts	100.00% <100.00%> (ø)
...ages/snaps-controllers/src/snaps/SnapController.ts	97.53% <100.00%> (+0.05%)	⬆️
...ges/snaps-simulator/src/features/manifest/sagas.ts	100.00% <100.00%> (ø)
...naps-simulator/src/features/manifest/validators.ts	84.37% <100.00%> (ø)
...ages/snaps-simulator/src/features/polling/sagas.ts	94.33% <100.00%> (+0.86%)	⬆️
...s/snaps-simulator/src/features/simulation/slice.ts	87.34% <100.00%> (+1.04%)	⬆️
packages/snaps-utils/src/auxiliary-files.ts	100.00% <100.00%> (ø)
packages/snaps-utils/src/manifest/manifest.ts	99.30% <100.00%> (+0.06%)	⬆️
packages/snaps-utils/src/manifest/validation.ts	100.00% <100.00%> (ø)
... and 6 more

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