feat: add GetFingerprints & GetJsonFingerprints for primary and sub keys

[Jun-Amane]

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• GetFingerprints() []string Returns SHA-1 fingerprints of primary key and all subkeys.

• GetJsonFingerprints() (string, error) Same data, JSON-encoded for gomobile bind where slice-of-string is not always stable across language bindings.

This allows apps to list every fingerprint including subkeys without re-implementing the walk logic outside the library.

Unit test included (crypto/key_test.go).