PeerManager: move the check for banned peers from connection_established (#4569)

## Issue Addressed
https://github.com/sigp/lighthouse/issues/4543
## Proposed Changes
- Removes `NotBanned` from `BanResult`, implements `Display` and `std::error::Error` for `BanResult` and changes `ban_result` return type to `Option<BanResult>` which helps returning `BanResult` on `handle_established_inbound_connection`  
- moves the check from for banned peers from `on_connection_established` to `handle_established_inbound_connection` to start addressing #4543.
- Removes `allow_block_list` as it's now redundant? Not sure about this one but if `PeerManager` keeps track of the banned peers, no need to send a `Swarm` event for `alow_block_list` to also keep that list right? 
 
## Questions

-  #4543 refers:
>  More specifically, implement the connection limit behaviour inside the peer manager.

@AgeManning do you mean copying `libp2p::connection_limits::Behaviour`'s code into `PeerManager`/ having it as an inner `NetworkBehaviour` of `PeerManager`/other? If it's the first two, I think it probably makes more sense to have it as it is as it's less code to maintain.

> Also implement the banning of peers inside the behaviour, rather than passing messages back up to the swarm.

I tried to achieve this, but we still need to pass the `PeerManagerEvent::Banned` swarm event as `DiscV5` handles it's node and ip management internally and I did not find a method to query if a peer is banned. Is there anything else we can do from here?

3397612160/beacon_node/lighthouse_network/src/discovery/mod.rs (L931-L940)

Same as the question above, I did not find a way to check if `DiscV5` has the peer banned, so that we could check here and avoid sending `Swarm` events

3397612160/beacon_node/lighthouse_network/src/peer_manager/network_behaviour.rs (L168-L178)

Is there a chance we try to dial a peer that has been banned previously? 

Thanks!

beacon_node/lighthouse_network/src/peer_manager/mod.rs

@@ -415,7 +415,7 @@ impl<TSpec: EthSpec> PeerManager<TSpec> {
     /// Reports if a peer is banned or not.
     ///
     /// This is used to determine if we should accept incoming connections.
-    pub fn ban_status(&self, peer_id: &PeerId) -> BanResult {
+    pub fn ban_status(&self, peer_id: &PeerId) -> Option<BanResult> {
         self.network_globals.peers.read().ban_status(peer_id)
     }
 
@@ -803,7 +803,7 @@ impl<TSpec: EthSpec> PeerManager<TSpec> {
     ) -> bool {
         {
             let mut peerdb = self.network_globals.peers.write();
-            if !matches!(peerdb.ban_status(peer_id), BanResult::NotBanned) {
+            if peerdb.ban_status(peer_id).is_some() {
                 // don't connect if the peer is banned
                 error!(self.log, "Connection has been allowed to a banned peer"; "peer_id" => %peer_id);
             }