Unstable Backend: add back the old chainHead RPCs/tests (#1137)

* add back unstable APIs and tests

* clippy fixes

* fmt with new rust

* deserialize from object or array into api versions

* remove inspect

* add some of the other unstable RPC methods we might want

* clippy fix

* no unused deps warning in test crate

* fix doc fails

* legacy -> unstable

Co-authored-by: Tadeo Hepperle <62739623+tadeohepperle@users.noreply.github.com>

* runtime_updates -> with_runtiem

* add test and be consistent with naming of unstable RPCs

---------

Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>
Co-authored-by: Tadeo Hepperle <62739623+tadeohepperle@users.noreply.github.com>

subxt/src/backend/mod.rs

@@ -8,6 +8,7 @@
 
 pub mod legacy;
 pub mod rpc;
+pub mod unstable;
 
 use crate::error::Error;
 use crate::metadata::Metadata;