tor-keymgr: Encode whether the key is public or private in the file extension.

There are 2 reasons to make this change:

• because having the word private in the extension will make it more difficult to accidentally misuse or misplace a private key (see !1618 (comment 2947461))
• because Keystores will soon grow a list() function returning all (ArtiPath, KeyType)s in the keystore, and in order for ArtiNativeKeystore to implement this function, it will need to be able to reverse the KeyType -> file extension mapping (if two different KeyTypes are mapped to the same extension, ArtiNativeKeystores won't be able to reverse the mapping)