Events API uses consistent error codes

[tomhjp]

A couple of small tweaks for consistency:

• UseProtoNames when serializing JSON fields to ensure we use underscores instead of proto3's default camel case to make us consistent with the rest of the Vault API. EDIT: Superseded by events: WS protobuf messages should be binary #19232 which changed to returning the cloud events formatted version of the event. I kept the additional test coverage though.
• Return 403 for unauthorized requests

For illustration on the status codes:

$ curl --fail -H "X-Vault-Token: $(vault token create -policy=default -field=token)" http://127.0.0.1:8200/v1/sys/mounts
curl: (22) The requested URL returned error: 403

$ curl --fail http://127.0.0.1:8200/v1/sys/mounts
curl: (22) The requested URL returned error: 403

[swenson]

LGTM