Add `signstar-config` crate to handle Signstar configs, administrative and non-administrative credentials on a Signstar host

Add signstar-config and accompanying integration tests.

.containers/Containerfile.integration-test

+FROM archlinux
+
+WORKDIR /test
+
+RUN pacman-key --init && pacman -Sy --needed --noconfirm archlinux-keyring && cat .env && source /test/.env && pacman -Syu --needed --noconfirm cargo cargo-nextest