Expand validator sets API with the rest of the events and some getters

We could've added a storage API, and fetched fields that way, except we want
the storage to be opaque. That meant we needed to add the RPC routes to the
node, which also simplifies other people writing RPC code and fetching these
fields. Then the node could've used the storage API, except a lot of the
storage in validator-sets is marked opaque and to only be read via functions,
so extending the runtime made the most sense.

substrate/node/src/rpc/mod.rs

@@ -16,7 +16,9 @@ use jsonrpsee::RpcModule;
 use sc_client_api::BlockBackend;
 use sc_transaction_pool_api::TransactionPool;
 
+mod utils;
 mod blockchain;
+mod validator_sets;
 mod p2p_validators;
 
 pub struct FullDeps<C, P> {
@@ -42,6 +44,7 @@ pub fn create_full<
 
   let mut root = RpcModule::new(());
   root.merge(blockchain::module(client.clone())?)?;
+  root.merge(validator_sets::module(client.clone()))?;
   if let Some(authority_discovery) = authority_discovery {
     root.merge(p2p_validators::module(id, client, authority_discovery)?)?;
   }