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/config/substrate.rs

@@ -71,7 +71,7 @@ impl<N, H> Header for SubstrateHeader<N, H>
 where
     N: Copy + Into<u64> + Into<U256> + TryFrom<U256> + Encode,
     H: Hasher + Encode,
-    SubstrateHeader<N, H>: Encode,
+    SubstrateHeader<N, H>: Encode + Decode,
 {
     type Number = N;
     type Hasher = H;