Proposal for KeyCrypterFactory and new EncryptionType

[oliveraemmer]

This PR proposes the implementation of a KeyCrypterFactory which allows users of the library to import their own KeyCrypters using their own EncryptionTypes. Currently this only works by extending the KeyCrypterScrypt class.

Change log:

• Add KeyCrypterFactory which can be set in WalletProtobufSerializer
• Add encryptionType field in EncryptedData
• Add Protos.Wallet.EncryptionType ENCRYPTED_KEYSTORE_AES in wallet.proto
• BasicKeyChain and DeterministicKeyChain: Check if one of the wallet encryption types is set
• DeterministicSeed: Return the encryptionType of the EncryptedData
• WalletProtobufSerializer: Use KeyCrypter provided by KeyCrypterFactory if the new encryption type ENCRYPTED_KEYSTORE_AES is defined in the protobuf