encryptor lacks sufficient typing #295

Currently, the encryptor class variable (and the corresponding constructor option) is typed with any.
As a controller client can inject its own encryptor methods, the minimum encryptor interface needed by the controller should be clear.

Moreover, the solution should take into account that:

• MetaMask extension and mobile use different encryptors
  • MetaMask extension uses the default encryptor value, provided by KeyringController, which is @metamask/browser-passworder
  • MetaMask mobile injects its own encryptor.
• When cacheEncryptionKey options is set to true, KeyringController needs an encryptor capable of exporting the encryption key string along with encrypted/decrypted strings. This is only needed by the extension, when using MV3
  • This means that when cacheEncryptionKey is set to true, the encryptor must implement additional methods specific for this