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/http_api/tests.rs

@@ -1,6 +1,7 @@
 #![cfg(test)]
 #![cfg(not(debug_assertions))]
 
+use crate::doppelganger_service::DoppelgangerService;
 use crate::{
     http_api::{ApiSecret, Config as HttpConfig, Context},
     Config, ForkServiceBuilder, InitializedValidators, ValidatorDefinitions, ValidatorStore,
@@ -85,16 +86,21 @@ impl ApiTester {
             Hash256::repeat_byte(42),
             spec,
             fork_service.clone(),
+            Some(Arc::new(DoppelgangerService::new(log.clone()))),
             log.clone(),
         );
 
+        validator_store
+            .register_all_in_doppelganger_protection_if_enabled()
+            .expect("Should attach doppelganger service");
+
         let initialized_validators = validator_store.initialized_validators();
 
         let context: Arc<Context<TestingSlotClock, E>> = Arc::new(Context {
             runtime,
             api_secret,
             validator_dir: Some(validator_dir.path().into()),
-            validator_store: Some(validator_store),
+            validator_store: Some(Arc::new(validator_store)),
             spec: E::default_spec(),
             config: HttpConfig {
                 enabled: true,