Keyring should have a fix encoding

[matteo-cristino]

Some keys in the keyring follow the CONF encoding during the output phase:

https://github.com/dyne/Zenroom/blob/d08d5162facbf3f6bf8d4bdcee5c4c72fff121cd/src/lua/zencode_keyring.lua#L186-L190

while in the input there is only a fixed encoding (for the above is base64). I think that generally it is better if the keyring encoding is fixed, so that we are alsways sure with what type of encoding we have to load the key.

[jaromil]

In cass we fix the encoding then base64 is preferred as requiring less changes, also is the most interoperable from shell scripts.