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>

Cargo.lock

@@ -15970,6 +15970,7 @@ dependencies = [
  "sp-consensus",
  "sp-core",
  "sp-maybe-compressed-blob",
+ "sp-rpc",
  "sp-runtime",
  "sp-version",
  "substrate-test-runtime",