Add support for wallet import via private key

Context

Currently forc wallet import only supports importing wallet(s) via mnemonic. Private keys are commonly used to more easily transfer accounts across wallets; hence it should also be supported here.

Current behaviour of forc wallet import:

> forc wallet import

Please enter your mnemonic phrase:  

Proposed behaviour:

> forc wallet import

Please enter your mnemonic phrase or private key:

This solution retains the simplicity of the current flow while supporting importing accounts via both mnemonic and private-key - without introducing additional flags.