Validator manager should accept 4 letter prefixes for mnemonic words

[karalabe]

Currently the validator manager when creating new validators requests the user to type in their mnemonic in full. Many cold wallets (i.e. the metal ones) only store the first 4 letters of each word as they uniquely identify the word. It would be nice if lighthouse would also accept 4 letter prefixes. Generally all tools that ingest mnemonics can handle this shortened notation.

[karalabe]

E.g. The same feature implemented on the upstream deposit CLI: ethereum/staking-deposit-cli#242

[daviesalex]

If for some reason supporting the 4 letter prefix isnt possible directly in the client, does it makes sense to change the "Invalid mnemonic" warning:

lighthouse/common/account_utils/src/lib.rs

Line 272 in 441fc16

eprintln!("Invalid mnemonic");

to a prompt to enter full words from the bip-0039 wordlist (https://github.com/bitcoin/bips/blob/master/bip-0039/english.txt)?