Add `ureq` backend to `blitz-net`

The motivation for this is primarily as a binary size optimisation for apps that want networking but do not need 100s of connections and do not otherwise need `tokio` or an async executor in their stack.

The implementation should:

- [ ] Be part of the existing `blitz-net` crate with feature flags for each backend
- [ ] Use the new 3.x version of `ureq`
- [ ] Split the `reqwest` and `ureq` implementations in separate files/modules
- [ ] Use a threadpool to allow for multiple concurrent requests
- [ ] Implement the `blitz_net::get_text` method as well as `blitz_net::Provider`