Implement `store add` in CLI

[alilleybrinker]

Right now, the CLI can only create new manifests directly in the store with the manifest create subcommand. We want it to also be able to add manifests which exist on the local file system but are not found in the manifest store.

The general process should look like:

• Checking if a file is a valid manifest by parsing it and storing it in memory.
• (If the user provided a target), modifying the in-memory manifest to include that target.
• Writing the manifest to storage.

The advantage of making it in memory and writing it out this way is that it will use the same existing code that ensures we update the target index to record the target of the manifest.

[alilleybrinker]

Done in 5d4b26f