fix: Sync PrivateKeyEncoding type and struct

[FrederikBolding]

Sync PrivateKeyEncoding type and struct with keyring-api. It was out of sync causing issues for certain Snaps.

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Note

Low Risk
Narrow type and validation allowlist expansion aligned with keyring-api; no auth or data-path changes.

Overview
Adds base32 as a supported private-key encoding in the Snaps SDK and utils so they match @metamask/keyring-api.

The PrivateKeyEncoding union in permissions.ts and the PrivateKeyEncodingStruct enum used by endowment:keyring capabilities validation in keyring.ts previously only allowed hexadecimal and base58. Snaps that declare base32 in import/export format capabilities were failing type or runtime checks until this sync.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.59%. Comparing base (22be130) to head (05c94ac).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4104   +/-   ##
=======================================
  Coverage   98.59%   98.59%           
=======================================
  Files         429      429           
  Lines       12495    12495           
  Branches     1976     1976           
=======================================
  Hits        12320    12320           
  Misses        175      175           

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