artifact-download-url customization

Currently we only support hardcoded support for github artifact download urls, which makes installers gated behind github. We can solve this by just letting the user specify. This should possibly support some amount of templating, as seen in [the config for cargo-binstall](https://github.com/cargo-bins/cargo-binstall/blob/main/SUPPORT.md).

It might also help [github enterprise users](https://github.com/axodotdev/cargo-dist/issues/185).

Theoretical usecases:

* `artifact-download-url = "{repo}/releases/downloads/{tag}/"` (what we currently do for github)
* `artifact-download-url = "https://mywebsite.com/{tag}/"`

Exposing things like "app-name" and "app-version" are sketchier with workspaces but we can maybe allow them for single-app workspaces?

Start by reading the existing artifact-download-url behavior and the cargo-binstall SUPPORT.md configuration linked in the issue. Compare the current GitHub URL with the proposed {repo} and {tag} examples, and review issue 185 for GitHub Enterprise context. Done means users can configure non-GitHub download URLs with clearly defined templating behavior.

Written by the indexing model from the issue text.