chainHead: Support multiple hashes for chainHead_unpin method (#2295)

This PR adds support for multiple hashes being passed to the
`chainHeda_unpin` parameters.

The `hash` parameter is renamed to `hash_or_hashes` per
https://github.com/paritytech/json-rpc-interface-spec/pull/111.

While at it, a new integration test is added to check the unpinning of
multiple hashes. The API is checked against a hash or a vector of
hashes.

cc @paritytech/subxt-team

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

substrate/client/rpc-spec-v2/src/chain_head/chain_head.rs

@@ -48,6 +48,7 @@ use sc_client_api::{
 use sp_api::CallApiAt;
 use sp_blockchain::{Error as BlockChainError, HeaderBackend, HeaderMetadata};
 use sp_core::{traits::CallContext, Bytes};
+use sp_rpc::list::ListOrValue;
 use sp_runtime::traits::Block as BlockT;
 use std::{marker::PhantomData, sync::Arc, time::Duration};
 
@@ -432,9 +433,16 @@ where
 	fn chain_head_unstable_unpin(
 		&self,
 		follow_subscription: String,
-		hash: Block::Hash,
+		hash_or_hashes: ListOrValue<Block::Hash>,
 	) -> RpcResult<()> {
-		match self.subscriptions.unpin_block(&follow_subscription, hash) {
+		let result = match hash_or_hashes {
+			ListOrValue::Value(hash) =>
+				self.subscriptions.unpin_blocks(&follow_subscription, [hash]),
+			ListOrValue::List(hashes) =>
+				self.subscriptions.unpin_blocks(&follow_subscription, hashes),
+		};
+
+		match result {
 			Ok(()) => Ok(()),
 			Err(SubscriptionManagementError::SubscriptionAbsent) => {
 				// Invalid invalid subscription ID.