Keystores should auto-generate certain keys

Motivated by the discussion at !1147 (comment 2898055)

If the configured key store directory doesn't have all the required keys, our key store implementation (ArtiNativeKeyStore/CTorKeyStore) needs to be ready to auto-generate them

1. Which keys needs to be auto-generated?
2. Arti's KeyMgr has multiple underlying key stores - which of those should we be autogenerating keys in?