feat: Add ThrottleLayer to Transport layers

[0xKitsune]

Closes #1636

This PR adds a ThrottleLayer for alloy providers to throttle outbound rpc requests. While the RetryLayer works great for for retrying requests that exceed RPC provider limits, the ThrottleLayer reduces the frequency that requests need to be retried.

async fn main() -> eyre::Result<()> {
// --snip--
 let requests_per_second = NonZeroU32::new(50).unwrap();

 let client = ClientBuilder::default()
    // Throttle RPC requests at 40 requests per second 
    .layer(ThrottleLayer::new(requests_per_second))
    // The RetryBackoffLayer can be stacked with the throttle layer to retry failed requests
    .layer(RetryBackoffLayer::new(10, 300, 330))
    .http(rpc_endpoint.parse()?);
 
    // Get 100_000 blocks concurrently
    let futures_unordered = FuturesUnordered::new();
    let block_number = provider.get_block_number().await?;
    for i in block_number - 100_000..block_number {
        let provider = provider.clone();
        futures_unordered.push(async move {
            provider
                .get_block(i.into(), BlockTransactionsKind::Hashes)
                .await
        });
    }

    // Await the results
    let res = futures_unordered.collect::<Vec<_>>().await;

    }

[yash-atreya]

Thanks for this!

Can we put this behind a feature flag since it adds a new dependency?

A test would be great as well, you can put it inprovider/trait.rs

[0xKitsune]

@yash-atreya Ive updated the PR to put the Throttle logic behind a feature flag as well as added tests.

Note that I changed the ThrottleLayer::new() function to take a u32 instead of NonZeroU32 for simplicity. If 0 is passed to this function it will panic. I can revert this change back to use NonZeroU32 if preferred.

Lmk if you would like any other updates.