breaking: Replace reqwest with ureq.

[milesj]

Swaps reqwest for ureq behind the url feature. ureq is blocking, but it never builds a
runtime of its own, so loading a URL from inside an async runtime now blocks the calling thread
instead of panicking.

# reqwest::blocking, called from inside #[tokio::main]
thread 'main' panicked at tokio/src/runtime/blocking/shutdown.rs:51:21:
Cannot drop a runtime in a context where blocking is not allowed.

That panic is what an earlier async-loading experiment on this branch was working around. Switching
clients removes the need for it, so the loader stays entirely synchronous.

Fixes HTTPS for everyone downstream

reqwest was declared default-features = false with only blocking, which leaves it without a
TLS backend. The rustls feature was only present in dev-dependencies, so HTTPS worked in this
repository — where Cargo unifies the two — and nowhere else.

A standalone crate depending on schematic with features = ["config", "url"] fails on any HTTPS
request before this change and loads fine after it. Since is_secure_url rejects anything but HTTPS
outside of loopback, the feature was effectively unusable as published. ureq enables rustls by
default.

Behaviour changes

• ConfigError::ReadUrlFailed now carries a ureq::Error rather than a reqwest::Error. Breaking
  for anyone matching on it.
• A non-2xx response is now a ReadUrlFailed. reqwest returned Ok regardless of status, so a
  404 previously handed its error page to the parser and surfaced as a confusing parse error.

Dependencies

Net 47 fewer packages — the tokio, hyper, and h2 stack drops out, taking roughly 540 lines of
Cargo.lock with it.

Tests

Three added to url_sources_test: loading from inside an async runtime, which pins the property
this swap exists for; the 404 case; and the existing network-backed suite continues to pass
unchanged. tokio is a dev-dependency solely to drive the first one.

Also corrects a stray claim in the book that the url feature is enabled by default — it is not,
and never has been.

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