fix: store pubsubfrontend clone in rpcinner

[mattsse]

closes #1972

this is a bit cursed.
It described this in the docs but due to how:

alloy/crates/rpc-client/src/builder.rs

Line 44 in bb77d2f

RpcClient::new(self.builder.service(transport), is_local)

creates the transport service object if there's an additional layer, this downcast does not work:

alloy/crates/rpc-client/src/client.rs

Lines 211 to 212 in bb77d2f

	pub fn pubsub_frontend(&self) -> Option<&alloy_pubsub::PubSubFrontend> {
	self.transport.as_any().downcast_ref::<alloy_pubsub::PubSubFrontend>()

the solution to this is a workaround that captures the Pubsubfrontend before we collapse the service builder.

this can still be bypassed manually via

alloy/crates/rpc-client/src/client.rs

Line 163 in bb77d2f

pub fn new(t: impl IntoBoxTransport, is_local: bool) -> Self {

but all the builtin transport fn should now capture the pubsubfrontend.
pubsubfrontend is just a channel so capturing an additional clone seems fine

[gakonst]

nit: no need for this

Suggested change

	let provider = ProviderBuilder::new().disable_recommended_fillers().on_client(rpc_client);
	let provider = ProviderBuilder::new().on_client(rpc_client);

also i wish we had the ability to configure layers on the ProviderBuilder w/o having to drop to RpcClient, so you'd just do

Providerbuilder::new().layer(retry_layer).on_builtin(url).await?

[DaniPopes]

lgtm, the only thing would be that it creates a copy of the internal state instead of sharing it, so we might want to arc pubsubfrontend internally?

[mattsse]

hat it creates a copy of the internal state instead of sharing

can you elaborate on this? unclear what state you mean

[DaniPopes]

the Pubsubfrontend struct itself is now both inside of transport and in the new field

[mattsse]

this is fine because this is just another copy of the sender half

alloy/crates/pubsub/src/frontend.rs

Lines 18 to 22 in 7fb7a51

	pub struct PubSubFrontend {
	tx: mpsc::UnboundedSender<PubSubInstruction>,
	/// The number of items to buffer in new subscription channels. Defaults to
	/// 16. See [`tokio::sync::broadcast::channel`] for a description.
	channel_size: AtomicUsize,

which also gets dropped when the transport drops