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>

common/eth2/src/lighthouse_vc/http_client.rs

@@ -642,6 +642,30 @@ impl ValidatorClientHttpClient {
         let url = self.make_gas_limit_url(pubkey)?;
         self.delete_with_raw_response(url, &()).await
     }
+
+    /// `POST /eth/v1/validator/{pubkey}/voluntary_exit`
+    pub async fn post_validator_voluntary_exit(
+        &self,
+        pubkey: &PublicKeyBytes,
+        epoch: Option<Epoch>,
+    ) -> Result<SignedVoluntaryExit, Error> {
+        let mut path = self.server.full.clone();
+
+        path.path_segments_mut()
+            .map_err(|()| Error::InvalidUrl(self.server.clone()))?
+            .push("eth")
+            .push("v1")
+            .push("validator")
+            .push(&pubkey.to_string())
+            .push("voluntary_exit");
+
+        if let Some(epoch) = epoch {
+            path.query_pairs_mut()
+                .append_pair("epoch", &epoch.to_string());
+        }
+
+        self.post(path, &()).await
+    }
 }
 
 /// Returns `Ok(response)` if the response is a `200 OK` response or a

common/eth2/src/lighthouse_vc/types.rs

@@ -144,3 +144,8 @@ pub struct UpdateGasLimitRequest {
     #[serde(with = "eth2_serde_utils::quoted_u64")]
     pub gas_limit: u64,
 }
+
+#[derive(Deserialize)]
+pub struct VoluntaryExitQuery {
+    pub epoch: Option<Epoch>,
+}