Add new validator API for voluntary exit (#4119)

## Issue Addressed

Addresses #4117 

## Proposed Changes

See https://github.com/ethereum/keymanager-APIs/pull/58 for proposed API specification.

## TODO

- [x] ~~Add submission to BN~~ 
  - removed, see discussion in [keymanager API](https://github.com/ethereum/keymanager-APIs/pull/58)
- [x] ~~Add flag to allow voluntary exit via the API~~ 
  - no longer needed now the VC doesn't submit exit directly
- [x] ~~Additional verification / checks, e.g. if validator on same network as BN~~ 
  - to be done on client side
- [x] ~~Potentially wait for the message to propagate and return some exit information in the response~~ 
  - not required
- [x] Update http tests
- [x] ~~Update lighthouse book~~ 
  - not required if this endpoint makes it to the standard keymanager API

Co-authored-by: Paul Hauner <paul@paulhauner.com>
Co-authored-by: Jimmy Chen <jimmy@sigmaprime.io>

validator_client/src/lib.rs

@@ -94,6 +94,7 @@ pub struct ProductionValidatorClient<T: EthSpec> {
     doppelganger_service: Option<Arc<DoppelgangerService>>,
     preparation_service: PreparationService<SystemTimeSlotClock, T>,
     validator_store: Arc<ValidatorStore<SystemTimeSlotClock, T>>,
+    slot_clock: SystemTimeSlotClock,
     http_api_listen_addr: Option<SocketAddr>,
     config: Config,
 }
@@ -461,7 +462,7 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
         let sync_committee_service = SyncCommitteeService::new(
             duties_service.clone(),
             validator_store.clone(),
-            slot_clock,
+            slot_clock.clone(),
             beacon_nodes.clone(),
             context.service_context("sync_committee".into()),
         );
@@ -482,6 +483,7 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
             preparation_service,
             validator_store,
             config,
+            slot_clock,
             http_api_listen_addr: None,
         })
     }
@@ -544,6 +546,7 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
                 graffiti_flag: self.config.graffiti,
                 spec: self.context.eth2_config.spec.clone(),
                 config: self.config.http_api.clone(),
+                slot_clock: self.slot_clock.clone(),
                 log: log.clone(),
                 _phantom: PhantomData,
             });