Add specs for API methods for more user preferences (identicon type, dark mode preference, metrics opt-in)

Dapps may want to have the following MetaMask preferences accessible via API:
- identicon type (jazzicon vs blockie)
- dark mode preference
- metrics opt-in

This would allow dapps to ensure config of their UX is consistent with the wallet config.

Start by reviewing the existing API method specifications and conventions in the MetaMask/api-specs repository. Determine how the three listed preferences—identicon type, dark mode, and metrics opt-in—should be represented and exposed, then confirm that each has a complete specification consistent with the existing API surface.

Written by the indexing model from the issue text.