feat: add capabilities to keyring endowment

[hmalik88]

Adding capabilities to the keyring endowment as part of keyring v2 work (https://github.com/MetaMask/decisions/blob/main/decisions/core/0006-keyring-interface.md)

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Note

Medium Risk
Expands the endowment:keyring permission/manifest surface with a new validated caveat, which could impact permission granting/validation and downstream keyring behavior if mis-specified or inconsistently interpreted.

Overview
Adds optional capabilities support to the endowment:keyring permission by introducing a new keyringCapabilities caveat, updating the endowment specification/mapper, and exporting getKeyringCaveatCapabilities to read it from permissions.

Updates @metamask/snaps-utils to include SnapCaveatType.KeyringCapabilities plus a new KeyringCapabilitiesStruct/assertIsKeyringCapabilities, and extends manifest validation and SDK InitialPermissions typings to accept a capabilities object under endowment:keyring. Tests and snapshots are updated accordingly (plus minor coverage threshold bump).

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[socket-security]

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👍 No dependency changes detected in pull request

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.56%. Comparing base (17136ba) to head (5f1946d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3903   +/-   ##
=======================================
  Coverage   98.55%   98.56%           
=======================================
  Files         425      425           
  Lines       12358    12381   +23     
  Branches     1935     1938    +3     
=======================================
+ Hits        12180    12203   +23     
  Misses        178      178           

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