This PR adapts the `archive_storage` implementation from a method to a
subscription.

This keeps the archive APIs uniform and consistent.

Builds on: https://github.com/paritytech/polkadot-sdk/pull/5997

cc @paritytech/subxt-team

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Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: James Wilson <james@jsdw.me>