feat: add BlockAnnounce trait for outbound block propagation

[itsdevbear]

This PR adds a symmetric counterpart to BlockImport for announcing blocks to the network. This is useful for custom chains that want to leverage a singular p2p layer instead of running a separate consensus client.

Scope:

• BlockAnnounce trait with poll-based interface
• Integration with NetworkManager's event loop
• Builder API for easy configuration
• PoS safety check to prevent invalid propagation
• Optional by default, backward compatible

Example Usage:

use reth_network::announce::{BlockAnnounce, BlockAnnounceRequest, PropagationStrategy};
use tokio::sync::mpsc;
use std::task::{Context, Poll};

#[derive(Debug)]
struct MyCustomBlockAnnouncer {
    rx: mpsc::UnboundedReceiver<(Block, B256)>,
}

impl BlockAnnounce<Block> for MyCustomBlockAnnouncer {
    fn on_announced_block(&mut self, _block: Block) {
        // do some stuff
    }

    fn poll(&mut self, cx: &mut Context<'_>) -> Poll<BlockAnnounceRequest<Block>> {
        match self.rx.poll_recv(cx) {
            Poll::Ready(Some((block, hash))) => {
                Poll::Ready(BlockAnnounceRequest::Announce {
                    block,
                    hash,
                    strategy: PropagationStrategy::Both, // Full block + hash
                })
            }
            _ => Poll::Pending,
        }
    }
}

// Usage:
let (to_announcer, from_sender) = mpsc::unbounded_channel();
let announcer = Box::new(MyCustomBlockAnnouncer { from_sender });

let config = NetworkConfig::builder(secret_key)
    .block_announce(announcer)
    .build(client);

// When you produce a block:
to_announcer.send((new_block, block_hash)).unwrap();
// NetworkManager will poll and announce it automatically, and then call the `on_announced_block` callback