Make storage_page_size for the LegacyBackend configurable

[jsdw]

Closes #1454 by allowing the user to configure the number of storage entries returned when iterating using the LegacyBackend. To make use of this you'll have to manually instantiate a backend etc, ie something like this:

// Connect to node:
let rpc_client = subxt::backend::rpc::RpcClient::from_insecure_url("ws://localhost:9944").await?;

// Use this connection to make calls using our legacy backend:
let backend = LegacyBackend::builder()
    .storage_page_size(1024)
    .build(rpc_client);

// Use the legacy backend to drive our Subxt client:
let client = OnlineClient::<PolkadotConfig>::from_backend(Arc::new(backend)).await?;

I added a builder just to mirror the UnstableBackend interface more closely, and to be a place where potentially we can expose more configuration if need be. It's definitely not ideal that you only have one global page size setting per client, but it's also a bit of a stop gap until we migrate to the new backend which streams storage entries and thus has no such configuration.

[tadeohepperle]

Looks good to me! :)