`om init`: Prepare for wider use

As a follow-up to #238 - fortify `om init` so that it can be used by various templates across repos.

- [x] Optimize registry loading: [over 1000 lines](https://github.com/juspay/omnix/blob/main/crates/omnix-init/registry/flake.lock) of `flake.lock` currently! #285
- [x] Downstream testing infra. When users define `params` on their flake, can we provide a way to test them via a test spec (à la `OmInitTest`)? #307
- [ ] Create a public registry of available templates #314

Aside:
- More templates
- https://github.com/srid/nixos-flake/tree/master/examples