wallet: Add import-file command

[matevz]

This PR:

• adds the new wallet import-file command for importing the existing keypair from the PEM file. This helps existing node operators to vote on the upcoming network upgrade, because the governance operations were deprecated in oasis-node and they are forced to use the Oasis CLI
• refactors the wallet command into its own subfolder
• adds Network-Account selectors admonitions to the network command as well

Fixes #140.

[kostko]

Would be cool to also have an example/test that performs the import.

[matevz]

Would be cool to also have an example/test that performs the import.

Testing the interactive commands is tricky. We could add --yes for import-file and other wallet (import, create) commands as well, but I don't think it's a good idea to encourage people having unencrypted accounts in their wallet. Let's leave this for another PR.

[kostko]

but I don't think it's a good idea to encourage people having unencrypted accounts in their wallet

Yeah we could just use it for tests but not include it in docs.