add silent initialization option

[dennisvang]

Initializing a repo from an existing .tufup-repo-config file is possible using the command line or api, but this may require user intervention (e.g. confirmation for directory creation).

For CI/CD workflows, it would be convenient to have a 'silent' initialization option for the CLI and API, which does not ask for user confirmation.

For example:

tufup init --silent

or

from tufup.repo import Repository
Repository.from_config().initialize(silent=True)

Note this will require some thought with regard to handling key-pair creation with passwords.