As a following up on #874, we should create a default config file.

The simplest idea i have is: payjoin-cli can have an additional boolean --set-config flag, that takes the config relevant clap arguments' values and saves them into the cargo.toml file (either the one in the current directory or the one in ~/.config/payjoin-cli, which should be the default if neither exists).

For ease of use/discoverability, a payjoin-cli configure command could take the same clap args as the other commands, and defaults config saving to true, but is otherwise a noop.

If no file exists, the example config file should be used but with the dummy values commented out (e.g. don't actually set rpcpassword = "password") before saving the given values.