feat: use ehttp instead of reqwest/tokio

[hiratara]

Hello,

I noticed that bevy_slippy_tiles uses a blocking API, which sometimes slows down my task pool. This PR replaces the reqwest crate with the ehttp crate to retrieve tiles asynchronously. I attempted to use reqwest for asynchronous operations, but it was too challenging due to tokio dependencies.

Please note that while ehttp supports WASM, this change does not include WASM support because we use multithreaded task pools and local file systems.

[edouardpoitras]

Great, thanks!

[hiratara]

Note that there are some discussions about which client we should use with IoTaskPool. ehttp may not be the best option, but I believe it is a good starting point.

bevyengine/bevy#6903

ehttp uses ureq on native, which has blocking I/O underneath and requires spawning a new thread for each request.

bevyengine/bevy#11694