Support all identity types (currently only private key is supported)

@raamb @vforvalerio87 
I have several comments. 
2. Only private-key identity is supported. Maybe we could directly use identity.py from snet-cli in order to support all identity types? At least we should add mnemonics identity, because we widely use it. 


_Originally posted by @astroseger in https://github.com/singnet/snet-sdk-python/pull/5#issuecomment-465615163_