Add RPC function state_getProof, resolves #1110 (#5649)

* Add RPC function state_getProof, resolves #1110

* Apply suggestions from code review

* Update client/rpc/src/state/state_full.rs

Co-Authored-By: Bastian Köcher <[email protected]>

* Update Cargo.lock

* Make block hash optional

* Wrap StorageProof as Bytes

* Add struct ReadProof

* Fix typo

Co-authored-by: Bastian Köcher <[email protected]>