Fix challenge deserialization for token-less challenge types

[bunert]

RFC 8555 only requires a token field for specific challenge types (http-01, dns-01, tls-alpn-01). Newer challenge types such as dns-persist-01 omit it. Add #[serde(default)] to the challenge token so deserialization does not fail when the field is absent.

[bunert]

Fixes #92

The high_level_tokio example is already broken:

cargo run --example high_level_tokio --features tokio -- \                                  ↵ 1
          -d your.domain.com \
          -p 443 \
          -c /tmp/acme-cache
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.11s
     Running `sudo -E target/debug/examples/high_level_tokio -d your.domain.com -p 443 -c /tmp/acme-cache`
2026-04-22T14:59:30.196Z INFO  [rustls_acme::incoming] event: AccountCacheStore
2026-04-22T14:59:33.442Z ERROR [rustls_acme::incoming] event: Order(Acme(Json(Error("missing field `token`", line: 28, column: 5))))
2026-04-22T14:59:37.801Z ERROR [rustls_acme::incoming] event: Order(Acme(Json(Error("missing field `token`", line: 22, column: 5))))
2026-04-22T14:59:43.724Z ERROR [rustls_acme::incoming] event: Order(Acme(Json(Error("missing field `token`", line: 28, column: 5))))
2026-04-22T14:59:51.607Z ERROR [rustls_acme::incoming] event: Order(Acme(Json(Error("missing field `token`", line: 22, column: 5))))

[FlorianUekermann]

Thank you!