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