wallet import: support MetaMask keys

[raulk]

There are two missing functionalities in wallet import:

1. It cannot import delegated keys exported by wallet export, thus breaking round tripping.
2. It does not support private keys exported from MetaMask.

Concretely, (2) is necessary to do low-level recovery from funky situations using Lotus directly (like sending low-level RBF messages).

[rjan90]

It cannot import delegated keys exported by wallet export, thus breaking round tripping.

This is supported today. Deleted t4-here:

lotus wallet delete t410fo7ng443xgufn4hjvkdgs6agiivutl35wq5ufzcy
Soft deleting address: t410fo7ng443xgufn4hjvkdgs6agiivutl35wq5ufzcy
Hard deletion of the address in `~/.lotus/keystore` is needed for permanent removal

Import by typing in private-key:

lotus wallet import                                             
Enter private key(not display in the terminal): 
imported key t410fo7ng443xgufn4hjvkdgs6agiivutl35wq5ufzcy successfully!

Import delegated key by path:

lotus wallet import /Users/phi/lotus/t4.key
imported key t410fo7ng443xgufn4hjvkdgs6agiivutl35wq5ufzcy successfully!

Afaict, importing MetaMask keys are not supported today. I have updated the headline to reflect the current state of lotus wallet import