Allow users to add account address to the keyring by providing private key instead of mnemonic phrase

[ltfschoen]

I want to add functionality to secretcli, so that users can add a specific account to the keyring on Secret localhost by providing the private key associated with that account. for example, i'd like to be able to do this:

secretcli keys add --recover-privatekey secret-4
  > Enter your bip39 mnemonic
  <ENTER_MNEMONIC_PHRASE>

Where secretcli keys add --help would be updated to include:

--recover-privatekey    Provide private key hex value to recover existing key instead of creating

At the moment it only allows users to provide the mnemonic seed phrase, as shown below, but that doesn't add the correct account address to the keyring. e.g.

secretcli keys add --recover secret-4
  > Enter your bip39 mnemonic
  <ENTER_MNEMONIC_PHRASE>