`rust-server-codegen`: fix deserialization of escaped query string data (#1058)

We are currently deserializing the query string into `Vec<(&str,
&str)>`. `serde_urlencoded` panics if the input string slice contains
escaped data, since in that case it needs to allocate a new `String` to
unescape the input string slice's contents.

Instead of deserializing to `Vec<(String, String)>`, we can instead use
`Cow<'a, str>` so that deserialization only allocates when strictly
required.

Reference: https://github.com/serde-rs/serde/issues/1413#issuecomment-494892266