Add at_latest() to use instead of .at(None) in various places.

[jsdw]

Currently we have APIs like:

let client = OnlineClient::<PolkadotConfig>::new();

// fetch latest block:
let block = client.blocks().at(None).await?;
// fetch latest events:
let events = client.events().at(None).await?;
// fetch latest storage entries:
let storage = client.storage().at(None).await?;

Fetching the latest block/event/storage seems like a common need, and moreso, we only need the async/await + Result for one of the paths (ie when None is provided). So how about we split into two code paths:

let client = OnlineClient::<PolkadotConfig>::new();

// fetch latest block:
let block = client.blocks().at_latest().await?;
// fetch block at `block_hash`:
let block = client.blocks().at(block_hash); // no more Option

// fetch latest events:
let events = client.events().at_latest().await?;
// fetch events at `block_hash`:
let events = client.events().at(block_hash); // no more Option

// fetch latest storage entries:
let storage = client.storage().at_latest().await?;
// fetch storage at block hash:
let storage = client.storage().at(block_hash); // no more Option

That way, the interface is a bit clearer, and we avoid unnecessary result unwrapping etc where it's not needed (ie somebody could get a block hash via client.blocks().at_latest().await?.hash() and then pass that to subsequent calls and avoid the awaiting/unwrapping.

[tadeohepperle]

@jsdw I created a PR for this here: #900
Unfortunately I think the alleviation of async + Result is only possible for the storage_client.at(hash) function, not for the other two, because they still make rpc requests in case the hash is already known. So for the blocks_client and the events_client I just created wrapper functions that make the API simpler.