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/api.rs

@@ -21,6 +21,7 @@
 //! API trait of the chain head.
 use crate::chain_head::event::{FollowEvent, MethodResponse, StorageQuery};
 use jsonrpsee::{core::RpcResult, proc_macros::rpc};
+use sp_rpc::list::ListOrValue;
 
 #[rpc(client, server)]
 pub trait ChainHeadApi<Hash> {
@@ -109,16 +110,22 @@ pub trait ChainHeadApi<Hash> {
 		call_parameters: String,
 	) -> RpcResult<MethodResponse>;
 
-	/// Unpin a block reported by the `follow` method.
+	/// Unpin a block or multiple blocks reported by the `follow` method.
 	///
 	/// Ongoing operations that require the provided block
 	/// will continue normally.
 	///
+	/// When this method returns an error, it is guaranteed that no blocks have been unpinned.
+	///
 	/// # Unstable
 	///
 	/// This method is unstable and subject to change in the future.
 	#[method(name = "chainHead_unstable_unpin", blocking)]
-	fn chain_head_unstable_unpin(&self, follow_subscription: String, hash: Hash) -> RpcResult<()>;
+	fn chain_head_unstable_unpin(
+		&self,
+		follow_subscription: String,
+		hash_or_hashes: ListOrValue<Hash>,
+	) -> RpcResult<()>;
 
 	/// Resumes a storage fetch started with `chainHead_storage` after it has generated an
 	/// `operationWaitingForContinue` event.