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`:
```sh
> forc wallet import

Please enter your mnemonic phrase:
```

## Proposed behaviour:

```sh
> 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.