Support for HTTP proxy: replace the HTTP client with reqwest

[sandhose]

This replaces the old stack of tower layers with just reqwest. This has many advantages, like:

• sane defaults in terms of redirects, timeouts, connection pooling
• HTTP_PROXY support
• removing a LOT of code we had to use our custom clients with Sentry and OpenTelemetry
• just a better user-facing API

I also took the opportunity to simplify A LOT mas-oidc-client to only the things we use, instead of making it so generic, which removes a lot of code

The only downside I see, is that it's a lot harder to test fake client requests. wiremock works well, but can't listen for HTTPS traffic.

One important thing to shout about in the changelog, is that we now support proxies, using the following environment variables:

• http_proxy/HTTP_PROXY for proxying HTTP requests
• https_proxy/HTTPS_PROXY for proxying HTTPS requests (note that this won't bypass CA verification, so stuff like mitmproxy require trusting an extra CA)
• all_proxy/ALL_PROXY for both
• no_proxy/NO_PROXY to add exclusions of hosts/IPs which shouldn't be proxied

It supports both HTTP and SOCKS5 proxies.

[cloudflare-workers-and-pages]

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