Restore non-hd keys generation functionality

[karbyshev]

This restores option for generation of random non-hd keys which was removed in #2158

• [ ] I have added a changelog
• [x] Git history is in acceptable state

[sug0]

I think this should be handled separately, with a cmdline flag that would conflict with --hd-path (e.g. --no-mnemonic). dispatching on the presence of --hd-path results in a bit of a strange UX, IMO

btw, we should also allow importing raw secret keys. in the current release (v0.26.0) we only support importing keys derived from input mnemonics.

[karbyshev]

Integrated in #2260