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

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