Support archive_unstable_call

[Tbaut]

While all the archive_unstable_ were quite straightforward so far, I'd like to support another call we're using in Multix archive_unstable_call which we use to call the metadata_metaddata at a specific block hash:

client
  ._request<{
    result: HexString
  }>('archive_unstable_call', [blockHash, 'Metadata_metadata', ''])
  .then((x) => opaqueMetadata(x.result)[1])

Here's the implementation: paritytech/polkadot-sdk#1582

[Tbaut]

Any pointer for this one?

[xlc]

looks like just an alias of state_call?