cargo: add missing feature "blocking", since webhook challenges uses …

…blocking reqwest

tldr, 5ba6dc8 upgraded reqwest from 0.11 -> 0.12. Reqwest fixed optional
dependencies internally to make deps _actually_ optional, which then
uncovered the mistake here of the missing feature-flag.

see: https:/seanmonstar/reqwest/releases/tag/v0.12.0

Cargo.toml

@@ -25,7 +25,7 @@ vaultrs-login = "0"
 url = "2"
 chrono = "0"
 num-traits = "0"
-reqwest = { version = "0", features = ["json"] }
+reqwest = { version = "0", features = ["blocking", "json"] }
 
 [profile.release]
 panic = "abort"