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/signing_method.rs

@@ -47,6 +47,7 @@ pub enum SignableMessage<'a, T: EthSpec, Payload: AbstractExecPayload<T> = FullP
     },
     SignedContributionAndProof(&'a ContributionAndProof<T>),
     ValidatorRegistration(&'a ValidatorRegistrationData),
+    VoluntaryExit(&'a VoluntaryExit),
 }
 
 impl<'a, T: EthSpec, Payload: AbstractExecPayload<T>> SignableMessage<'a, T, Payload> {
@@ -67,6 +68,7 @@ impl<'a, T: EthSpec, Payload: AbstractExecPayload<T>> SignableMessage<'a, T, Pay
             } => beacon_block_root.signing_root(domain),
             SignableMessage::SignedContributionAndProof(c) => c.signing_root(domain),
             SignableMessage::ValidatorRegistration(v) => v.signing_root(domain),
+            SignableMessage::VoluntaryExit(exit) => exit.signing_root(domain),
         }
     }
 }
@@ -203,6 +205,7 @@ impl SigningMethod {
                     SignableMessage::ValidatorRegistration(v) => {
                         Web3SignerObject::ValidatorRegistration(v)
                     }
+                    SignableMessage::VoluntaryExit(e) => Web3SignerObject::VoluntaryExit(e),
                 };
 
                 // Determine the Web3Signer message type.