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/http_metrics/metrics.rs

@@ -88,6 +88,11 @@ lazy_static::lazy_static! {
         "Total count of attempted SyncSelectionProof signings",
         &["status"]
     );
+    pub static ref SIGNED_VOLUNTARY_EXITS_TOTAL: Result<IntCounterVec> = try_create_int_counter_vec(
+        "vc_signed_voluntary_exits_total",
+        "Total count of VoluntaryExit signings",
+        &["status"]
+    );
     pub static ref SIGNED_VALIDATOR_REGISTRATIONS_TOTAL: Result<IntCounterVec> = try_create_int_counter_vec(
         "builder_validator_registrations_total",
         "Total count of ValidatorRegistrationData signings",