feat: pending logs subscription

[allnil]

draft for pending logs subscription #5617

[mattsse]

this is exactly what we want yes.

Atm this would only support the pending block received by the engine.

I believe we also want to support the locally built pending block by EthApi
what we can do here is to also add a watch channel and set the new pending block there:

https://github.com/paradigmxyz/reth/blob/main/crates/rpc/rpc/src/eth/api/mod.rs#L339-L342

then we can a new wrapper type that combines both watchers, the from the CL and the local one

[mattsse]

okay, overall this is pretty close to the functionality we want,

with watch channels and on demand receivers, there's no need to create them outside the struct that's holding the sender, because new receivers can be created on demand, so we want this to happen inside the new function instead.

struct A {watch::Sender}

impl A {

fn new() {
  Self {watch::channel()}
}

fn listener(&self) -> Receiver {
  self.watcher.subscribe()
}

}

[mattsse]

this is great!

this is now blocked by #6104

[mattsse]

sorry, this went stale but with the current design this is slightly difficult, we can solve this much easier now