Get a vault's public key for browser-encrypted credential values

Returns the public key that custom credential collection web apps use to encrypt values in the browser. Use this only if you run your own credential collection web app and want values encrypted in the browser, sent to your backend still encrypted, and forwarded to Kernel's API still encrypted. In every other case, including server-side code that already holds the plaintext, use value. The page encrypts each value to this key, your backend forwards the ciphertext unchanged as encrypted_value when creating or updating a credential item in this vault, and Kernel decrypts it.