[Feature]: Support display mnemonic discreetly for keys add

[Halimao]

Summary

simd keys add name will show mnemonic in output, we can show it discreetly on an alternate screen.

Problem Definition

As the default behaviour of simd keys add name will show the mnemonic, it would be more safe if we display mnemonic on an alternate screen, even through mnemonic print could be disabled by --no-backup.

Proposed Feature

After we run simd keys add test, the mnemonic will show on an alternate screen, with some prompt msg. With 'Enter' key press, will bring user back to original screen, and show the key info.