[Keystore] Allow keystore to select different encryption algorithms

[y3fers0n]

Allow keystore for web extension to select different encryption algorithms i.e.: aes-256-ctr and maybe salt

[hewigovens]

The changes I can think of:

1. Need to add a new method with AES key length parameter (128/192/256) to TWStoredKey
2. Fix the hardcode value aes-128-ctr in EncryptionParameters.cpp
3. Change Swift / TypeScript Keystore accordingly
4. Tests