Peer manager cfg (#2766)

## Issue Addressed
I've done this change in a couple of WIPs already so I might as well submit it on its own. This changes no functionality but reduces coupling in a 0.0001%. It also helps new people who need to work in the peer manager to better understand what it actually needs from the outside

## Proposed Changes

Add a config to the peer manager

beacon_node/lighthouse_network/src/peer_manager/config.rs

@@ -0,0 +1,43 @@
+/// The time in seconds between re-status's peers.
+pub const DEFAULT_STATUS_INTERVAL: u64 = 300;
+
+/// Default ping interval for outbound connections, in seconds.
+pub const DEFAULT_PING_INTERVAL_OUTBOUND: u64 = 15;
+
+/// Default interval for inbound connections.
+pub const DEFAULT_PING_INTERVAL_INBOUND: u64 = 20;
+
+/// Default number of peers to connect to.
+pub const DEFAULT_TARGET_PEERS: usize = 50;
+
+/// Configurations for the PeerManager.
+#[derive(Debug)]
+pub struct Config {
+    /* Peer count related configurations */
+    /// Whether discovery is enabled.
+    pub discovery_enabled: bool,
+    /// Target number of peers to connect to.
+    pub target_peer_count: usize,
+
+    /* RPC related configurations */
+    /// Time in seconds between status requests sent to peers.
+    pub status_interval: u64,
+    /// The time in seconds between PING events. We do not send a ping if the other peer has PING'd
+    /// us within this time frame (Seconds). This is asymmetric to avoid simultaneous pings. This
+    /// interval applies to inbound connections: those in which we are not the dialer.
+    pub ping_interval_inbound: u64,
+    /// Interval between PING events for peers dialed by us.
+    pub ping_interval_outbound: u64,
+}
+
+impl Default for Config {
+    fn default() -> Self {
+        Config {
+            discovery_enabled: true,
+            target_peer_count: DEFAULT_TARGET_PEERS,
+            status_interval: DEFAULT_STATUS_INTERVAL,
+            ping_interval_inbound: DEFAULT_PING_INTERVAL_INBOUND,
+            ping_interval_outbound: DEFAULT_PING_INTERVAL_OUTBOUND,
+        }
+    }
+}