manager: Enhance connection lifecycle with custom connection policies

[lexnv]

This PR makes the litep2p more generic wrt implementing custom connection limits and policies.

• the ConnectionMiddleware trait is introduced to offer the ability for developers to implement custom connection policies that hook directly into the connection lifecycle
• the trait offers the ability to reject connections before or after the negotiation process
• the ConnectionLimits object is refactored to implement the middleware trait
• the TransportEvent::PendingInboundConnection is enhanced with the socket address of the remote endpoint

A new middleware trait, named ConnectionMiddleware is introduced:

pub trait ConnectionMiddleware: Send {
   fn outbound_capacity(&mut self, peer: PeerId) -> crate::Result<usize>;

    fn check_inbound(
        &mut self,
        connection_id: ConnectionId,
        address: SocketAddr,
    ) -> crate::Result<()>;

    fn can_accept_connection(&mut self, peer: PeerId, endpoint: &Endpoint) -> crate::Result<()>;

    fn on_connection_established(&mut self, peer: PeerId, endpoint: &Endpoint);

    fn on_connection_closed(&mut self, peer: PeerId, connection_id: ConnectionId);
}

cc @paritytech/networking