Enhancement: Support repositories with empty password

Restic previously required a password to create or operate on repositories.
Using the new option `--insecure-no-password` it is now possible to disable
this requirement. Restic will not prompt for a password when using this option.

For security reasons, the option must always be specified when operating on
repositories with an empty password, and specifying `--insecure-no-password`
while also passing a password to restic via a CLI option or environment
variable results in an error.

The `init` and `copy` commands add the related `--from-insecure-no-password`
option, which applies to the source repository. The `key add` and `key passwd`
commands add the `--new-insecure-no-password` option to add or set an empty
password.

https://github.com/restic/restic/issues/1786
https://github.com/restic/restic/issues/4326
https://github.com/restic/restic/pull/4698
https://github.com/restic/restic/pull/4808
