Extend the `gitoid` crate to support `tree` objects

[alilleybrinker]

In the full Git object system, there are four kinds of objects: tree, commit, tag, and blob. They mean roughly the following:

• blob: A single file on the file system
• tree: A directory containing files on the file system
• commit: A commit object in the Git commit history
• tag: A tag object pointing to a specific commit

Right now, all of these are represented in an enum in the gitoid crate, but we don't really properly support any object besides the blob object.

I'd like to extend gitoid to at least also support tree objects, which would then permit encoding an entire file system tree hierarchy with the gitoid crate to use alongside attestations of file and directory-specific measurements.

With just blob, we only have the ability to identify individual files, but not to locate them within a structure. tree objects add that location!

[alilleybrinker]

I'd say this is now a non-goal, though if someone wants to add it in the future they're free to do so. I've documented that the gitoid crate is primarily for our own cases, and that's good enough for me!