Optionally Save Wallet Password on Web Onboarding

[rauljordan]

What type of PR is this?

Feature

What does this PR do? Why is it needed?

cc @dapplion

End Goal:

• To allow validator clients to launch with --web without requiring any prompt interaction
• To not need to manage individual --wallet-password-file files for validators on dappnode machines

Approach:

• On rpc.WalletCreate() which is called from the frontend during the onboarding flow, store the wallet password as a text file with standard user read permissions inside of the wallet directory (the directory already has safe permissions) and let the user know the file was written there
• On validator --web start, check if the user has a default wallet password in the wallet directory, and use that to unlock the validator keys and start things up. This refactoring is done inside of validator/node/node.go as that's where the logic of initializing the wallet occurs on startup.
• No changes need to be made to the UI for this feature
• Gate the feature behind the validator flag: --write-wallet-password-on-web-onboarding

Which issues(s) does this PR fix?

Fixes #7929