Doppelganger detection (#2230)

## Issue Addressed

Resolves #2069 

## Proposed Changes

- Adds a `--doppelganger-detection` flag
- Adds a `lighthouse/seen_validators` endpoint, which will make it so the lighthouse VC is not interopable with other client beacon nodes if the `--doppelganger-detection` flag is used, but hopefully this will become standardized. Relevant Eth2 API repo issue: https://github.com/ethereum/eth2.0-APIs/issues/64
- If the `--doppelganger-detection` flag is used, the VC will wait until the beacon node is synced, and then wait an additional 2 epochs. The reason for this is to make sure the beacon node is able to subscribe to the subnets our validators should be attesting on. I think an alternative would be to have the beacon node subscribe to all subnets for 2+ epochs on startup by default.

## Additional Info

I'd like to add tests and would appreciate feedback. 

TODO:  handle validators started via the API, potentially make this default behavior

Co-authored-by: realbigsean <seananderson33@gmail.com>
Co-authored-by: Michael Sproul <michael@sigmaprime.io>
Co-authored-by: Paul Hauner <paul@paulhauner.com>

validator_client/src/config.rs

@@ -47,6 +47,9 @@ pub struct Config {
     pub http_metrics: http_metrics::Config,
     /// Configuration for sending metrics to a remote explorer endpoint.
     pub monitoring_api: Option<monitoring_api::Config>,
+    /// If true, enable functionality that monitors the network for attestations or proposals from
+    /// any of the validators managed by this client before starting up.
+    pub enable_doppelganger_protection: bool,
 }
 
 impl Default for Config {
@@ -76,6 +79,7 @@ impl Default for Config {
             http_api: <_>::default(),
             http_metrics: <_>::default(),
             monitoring_api: None,
+            enable_doppelganger_protection: false,
         }
     }
 }
@@ -264,6 +268,10 @@ impl Config {
             });
         }
 
+        if cli_args.is_present("enable-doppelganger-protection") {
+            config.enable_doppelganger_protection = true;
+        }
+
         Ok(config)
     }
 }