feat: Add alternate tags for user images (#172)

This adds the ability for the user to add alternate tags to their
images. These are designed to replace the `latest` and timestamp (e.g.
`20240429`) tags.

integration-tests/test-repo/recipes/recipe.yml

@@ -1,6 +1,9 @@
 name: cli/test
 description: This is my personal OS image.
 base-image: ghcr.io/ublue-os/silverblue-main
+alt-tags:
+  - gts
+  - stable
 image-version: 39
 modules:
   - from-file: akmods.yml