grandpa: Use storage proofs for Grandpa authorities (#3734)

* grandpa: Write Grandpa authorities to well known key.

Instead of requiring execution proofs for Grandpa authorities, this
enables much simpler storage proofs.

* grandpa: Introduce named AuthorityList type.

* grandpa: Storage migration for srml-grandpa module.

* Remove no-longer-used GrandpaApi runtime API.

* grandpa: Write AuthorityList to storage with encoding version.

We expect the AuthorityList type may change (eg. key changes). To make
upgrades smoother, include a version in the stored value.

* Bump node runtime spec version.

* Update srml/grandpa/src/lib.rs

Co-Authored-By: André Silva <[email protected]>