Wallet backup tool

[LesnyRumcajs]

Issue summary

There's currently no way of easily backing up all the wallet in Forest (I am not aware of a proper tool specifically for Lotus). To backup wallets one would have to either:

• backup the entire keystore file - this would also include the JWT key, which may be unwanted,
• export every wallet, one by one. It's not a huge hassle, but can be wrapped in a single command that would also deal with the recovery.

Acceptance criteria:

• [ ] The tool is node-agnostic - it should work both with Forest and Lotus. The way to achieve this is to use exclusively the existing Wallet API to compose a new command.
• [ ] All addresses are exported and saved into a file
• [ ] All addresses can be recovered from that file.
• [ ] Round-trip tests with edge cases (e.g., some keys already present),
• [ ] Documentation and how-to

Open question: should the exported file be encrypted via the tool, or is that something the user should take care of?

Other information and links