keys: add warning on stellar key for seed phrase shorter than 24

[leighmcculloch]

The stellar keys add command could play a role in user education by warning when encountering keys that were likely generated with poor entropy. This is usually not something that can be detected with keys, except seed phrases of a length shorter than 24 words.

I suggest we add a warning like this when entering a seed phrase that's 12 words long instead of 24:

$ stellar keys add me --seed-phrase
Type a 12 or 24 word seed phrase: 
word word word word word word word word word word word word
⚠️  Warning that 12 word seed phrases do not result in a key generated with sufficient entropy to be secure. It is safer to use a 24 word seed phrase where possible. To generate a new key, use the `stellar keys generate` command.

Depends on:

• keys: generate 24-word keys #1803