REST VC: Subscribe to Beacon API events (#13354)

* Initial code for head event streaming

* handle events and error

* keepalive event

* tests

* generate new mock

* remove single case select

* cleanup

* explain eventByteLimit

* use 2 channels in test

* review

* more review

---------

Co-authored-by: james-prysm <90280386+james-prysm@users.noreply.github.com>

validator/accounts/testing/mock.go

@@ -212,3 +212,7 @@ func (m *Validator) SetProposerSettings(_ context.Context, settings *validatorse
 	m.proposerSettings = settings
 	return nil
 }
+
+func (_ *Validator) StartEventStream(_ context.Context) error {
+	panic("implement me")
+}